How do you tell if other guys are gay?

I went to a movie with a dude today, and I really enjoyed it, but I guess I was kind of hoping that he had asked me out on a date, and while he was perfectly polite it seemed like he just wasn’t interested in anything like that.I guess my question is whether there’s a way to tell if he’s gay but not into me, or just straight and wanted to catch a movie or what.Anyway, any and all advice would be welcome.