aka cottaging. Though the early Internet had sites aimed at it (I have no idea if cottaging.co.uk is still going) where you could see what someone was packing before hand.
Fabswingers has been around since... 2006 (apparently). And obviously you had swinger clubs where what was on offer was seen.
And local ad sites like GumTree and Vivastreet.
Point being, not only was there very much ways of knowing but they're still going today.
(Fabswingers, while catering for everyone is mostly cis couples but it does also has a spin off site - Fabguys, aimed more at gay men and trans people - basically more focused on people looking for dick).
And you had the classic text message of "So and so gave me your number, they said you had a big dick" - which was always nice.
Always a sad time when you're out and headed back for a MFM or something and someone was way under measure. Like, just let the person know. When they're obviously a size-queen etc you're just walking into an awkward "I've changed my mind" conversation.