Unfortunately we live in such turbulent times that there are a lot of people will do anything to get rich. Sometimes they pose as Nigerian princes or send you phising links, but sometimes they also sell you spiritual relief and bullshit rituals.
My family was one of such victims. This scammer was parading as a Tantrayana/Vajrayana monk, and it took me years to realize that he was fake because that first time I met him I was a snot-faced teenagers and I believed what my parents believed and because his patrons were rich people.
But looking back there were glaring signs that he was fake. Not all of these points will apply across the world. I think that my experience might be specific to Chinese-Indonesians being quite vulnerable to exploitation because of our minority status and history of oppression. So do keep that in mind and always use critical thinking. If something feels wrong, then do your research and ask other people for opinions.
1) Real monks will never sell you things
Whether it's physical items such as beads and holy water, or a 'service' like a cleansing ritual and empowerment, they will not ask for money. Donations could be welcome depending on the school/vihara (e.g. Mahayana doesn't see money as evil). They should be giving blessings without expecting anything in return.
The fake monk in my experience sold a 'prosperity ritual', and he worded it in such a way that my dad's business wouldn't survive without it. He 'asked' for IDR 20 million (abt 1200 USD, but since this is Global South, the money means much more to us). My dad stopped the rituals some years ago, and you know what? We survived the pandemic. The business is still going. So obviously, he was bullshitting.
2) Fake monks favor rich patrons
The fake monk traveled from city to city, which by itself is fine. But whenever he was around, he always stayed in 5⭐hotel.
3) Fake monks don't actually teach you Buddhism
It's kinda obvious when I say it out loud, but again, I will point at the fact that, usually these fake monks will target desperate people who are looking to spirituality as a means of escape or improve their living conditions. Such fake monks will tell you to 'pray to this Buddha and it will improve' without explaining who they are and what their story is like. A lot of times in Buddhism, you have to think and draw conclusions yourself from anecdotes and mythology.
I cannot express how annoyed I was whenever my dad told me to pray to X Buddha because the fake monk said so. When I asked him 'What is that Buddha's story', he couldn't answer.
4) Fake monks might look like they give a shit, but they actually don't
This one is harder to spot. It's in the little things, and usually, it comes in the form of their words. Those might take time to process, perhaps days, weeks, or months later you'd realize that 'wait a minute, something's not right'. In this case, always trust your gut.
I think the core of this also applies to other kinds of spiritual scammer. New Age 'gurus' or whatever they want to call themselves these days, will prey on vulnerabilities especially when we're experiencing a global recession now. People are more desperate for a way out, and that's when these scammers will swoop in. Do not trust anyone whose motivation is to drain your wallet.