One of the biggest tells for me is someone being cruel toward other fans, especially people they've fallen out with. A little griping behind friendslock or trying to explain what's happened, that's normal, that's fine, but being vicious and mean towards them years on, that's a sure sign of a toxic person.

Agreed! One of my red flags is when people persistently treat other people as only valuable when they do something for them, when these others act as the scammer wants them to act. To me, that fits into a larger pattern with fannish grifters: they are the deities of the universe - whether real or fictional. The rest of us are non-player characters.