Glib Answer: Become a massage therapist! This is one of the main things our schooling is supposed to allow us to suss out.

Actual Answer: Nah but for real though you COULD do worse than to ask a massage therapist (or physiotherapist) "is this pain something that massage would help with?" It's also useful to distinguish between "pain that MASSAGE can help with" and "pain that a MASSAGE THERAPIST can help with", since massage therapists can help with pain in ways other than just rubbing stuff. I think the most useful way I can answer this is by running down a bunch of common types of pain (and some less-common ones, just for fun) and then saying what a massage therapist might do or recommend for them.

• Dull, aching pain that is somewhat relieved with movement or changes in position: usually indicative of muscle fatigue or stiffness from being in a position for too long. Massage is helpful, and massage therapists might recommend behavior modifications like stretching breaks or a more regular exercise routine, as well as specific stretches to target involved muscles.
• Sharp pain at a specific point following recent exertions that is relieved with rest: usually indicates a muscle strain or ligamentous sprain (easy way to remember the difference: remember that T stands for tendon). No massage right at the site of injury, but massage therapist might massage structures above the injury site to relax them, and will recommend methods of symptom relief like heat or ice, as well as giving you an idea of how long healing might take and some rehabilitative exercises.
• Pain that feels like burning, or electrical zapping, or numb, or tingling: usually indicates some kind of nerve involvement. Massage MIGHT help, if the problem is a peripheral nerve being irritated by a nearby muscle that's crowding it. Therapist might also give stretches or demonstrate some nerve-flossing techniques, and recommend certain positional adjustments if they suspect specific things might be contributing to nerve irritation (a common one is long-haul truckers keeping wallets in their back pockets while seated, which can lead to irritation of the sciatic nerve, leading to pain/numbness/tingling down the back of the leg).
• Sickening pain following some kind of impact: usually indicates a bone fracture. Massage can't help, go to the hospital. We'll see you when the cast comes off.
• Headache pain: depends on the kind of headache. If it's a tension headache - that is, a headache being partially or wholly caused by fatigue in head and neck muscles - then yes and people really love us for this one. However, a lot of things cause headaches. Dehydration, sun exposure, malnutrition, those we can't massage away but can usually recommend ways to address. Sinus congestion, too. We see a fair number of post-concussion cases, which always have a number of things to address and you kind of just have to take them as they come. Migraines are a mixed bag too.
• Cramping pain: both types of cramps (cramps to skeletal muscles and menstrual cramping) are unfortunately not especially helped by massage. Skeletal muscle cramps you have to pretty much just ride out, after which light massage MIGHT help (but is honestly better avoided). Menstrual cramping, sadly, we can't do very much for other than helping the patient relax in a general sense.
• Most types of abdominal pain: can't do much. Exquisite right-lower-quadrant pain that's hot to the touch? Probably a burst appendix, go to a hospital. Sharp, burning pain right in the center of your chest after meals? Probably GERD or a hiatal hernia, talk to a doctor. Severe right-sided abdominal pain lasting hours following fatty meals? Probably gallstones, talk to a doctor. Stomachache with long-lasting constipation? Okay actually we CAN help with this one, believe it or not. Slow, circular stroking of the abdomen with a washcloth soaked in warm water for 10-15 minutes can promote large intestine motility and get things moving again.
• Pain in breasts associated with lactation: again, this is a weird one, but massage therapists can actually help with this. It IS actually possible to do very light massage to areas on and around breast tissue that will help unblock clogged milk glands, which can be quite painful. Although massage therapists do all get a brief grounding in breast massage, the ones that make it a regular part of their practice will usually advertise as such (typically they're also ones that will advertise as specializing in pregnancy massage).
• Pain arising from any number of chronic fatigue/autoimmune conditions including CEFS, fibromyalgia, lupus, POTS, and so on: sometimes! Sorta! Your mileage may vary! Honestly with these conditions the patient probably knows better than I do, so I follow their lead - that, or we're working together to figure out what works best.
• Pain arising from multiple sclerosis symptoms: technically yes; multiple sclerosis causes uncontrollable contraction of skeletal muscles leading to extreme pain and fatigue, which IS something massage can help with. It's just not enough.

As with most medical questions, the ultimate answer is: it depends. If you have a specific set of symptoms and would like to run them by me, feel free to do so in the comments, or message me on Discord.

This was an extremely funny ask to get in concert with Iro's that I just answered. Anyways.

Yup, there's nothing magical about it either way. The differences between the two classes of hardware are just the maximum amount of force produced, RPMs, size, and features like oscillation etc. Someone can correct me on this, but I'm almost positive the Hitachi Magic Wand was originally marketed as a back massager before its utility as a Pussy Obliterator became too overwhelming. Vibration is just one of those things that feels good against our nerve endings. I do think standard-issue cleaning would keep any kind of vibrating massager up to hygienic standards, and I think most people would be able to tell if a level of vibration was going to be harmful/unenjoyable in their vulnerable bits.

I think form factor is actually the biggest stumbling block here in terms of using one for the other's purpose. Your standard-issue vibrating dild might technically feel nice between your shoulderblades, but I don't think it's going to be very easy to get & keep it in position (you might have better luck using it on hands or feet). By the same token, a massage gun is roughly as large as a good-sized power drill, which I think would be pretty difficult to maneuver satisfyingly around the contours of one's junk.

Brother this is a VERY funny one to get in the context of another ask that I'm going to answer right after.

To actually answer you:

• I've talked about some of the limitations of self-massage in a previous ask, so I'll skip those for the most part
• A big factor with self-massage is leverage, which is to say, how can you apply a satisfying amount of force to a structure without needing to do a huge amount of work yourself (and without having to develop massage-therapist-like musculature)? It comes down to tools and positioning.
• As far as tools go, you can always get specialized things like those curved massage sticks or a vibrating/percussive device like a massage gun. But you could probably do almost as well just going down to a sporting goods store and picking up a golf ball, a tennis ball, and a hard rubber ball.
• Then we get to positioning - the important thing is using body weight to transmit force, which means putting one of your balls against a wall or on the floor, and then leaning/lying against it. If the sensation is too intense, try putting a towel or something similar over the ball first. From a seated position, you can take your hard rubber ball and easily roll it over your thighs or your forearms using easy, circular motions. Another good one: put your golf ball on the floor and then roll the sole of your foot around on top of it to give yourself a nice foot massage!
• The big precautions are force and time. Try to only spend about 3-5 minutes on any given area, and try to only apply so much force that you're in "scratching an itch" levels of pain, not getting into "holding breath/starting to sweat" levels.

Those are my quickest tips & tricks! If you have more specific questions feel free to hit me up in the comments, I'm happy to go into more detail on anything.

• The answer to the first question is boring but somethings are just boring: to get better at giving massages, your partner should seek instruction on giving good massages. This video I just found has some good tips for beginners, but honestly if they're really committed they should seek out a day/weekend course where someone can give them instruction in person because a lot of the time just getting info from videos will miss out on certain relevant basics. One easy thing they could do is get some massage lotion and use that the next time they give you a backrub - not only will it reduce uncomfortable friction between their hands and your skin, but it'll ALSO make them FEEL more professional (I like lotion better than oil as it absorbs into the skin without irritating pores, and is easier to manage).
• Giving feedback is a skill in and of itself! Here's my advice: tell your partner to start with light pressure and slow strokes (you can give a perfectly good, satisfying massage without ever causing pain, trust me on this). After they've spent a couple minutes warming up the area with light pressure, they can slowly start to increase their pressure, still using slow, rhythmic strokes. For your part, you can give them feedback/reassurance, letting them know that you're comfortable. You could both also stand (like most people could) to "bone" up on your knowledge of the musculoskeletal anatomy of the back and shoulders - I promise it isn't that hard! There's technically a lot of INDIVIDUAL muscles but in terms of ones that're super relevant to massage we're only talking about: trapezius, lev scap, rhomboids, the erector spinae group, delts, lats, teres group, infraspinatus, multifidus, QL, glutes, and the splenius muscles. Learn those, how the scapula works, the basic layout of the spine and ribcage, and congrats, you've got a good working knowledge of the massage-able contours of the human back!