Maybe it's just the fa/tg/uy in me talking, but I think the easy litmus test is whether a person's involvement in a hobby requires a real investment of work on their part.
The local game shop I frequent has a lot of "LOL SO NERDY XD" clientele, but in my area of interest it's pretty easy to tell who's genuine and who's not. You don't show up every Monday to autistically detail tiny models unless you're serious. So when someone new shows up to paint night, I just treat them as I would anyone else, and I pay attention to a couple things: are they still showing up after a couple of months? Do they show up even when they don't have their friends tagging along? Do they show up with their models primed/assembled/partially painted indicating that they put in prep work on their own time? If yes to any, then that's heavily in favor of them being genuine.
In the case of Star Wars specifically, I guess this ties in with what a couple others have said about digging into the EU and actively seeking out content that interests you. I also appreciate >>33509 that it's about having a pure enthusiasm for something, but that's a lot harder gauge as an observer.