Yes, there is some, but very little above chance accuracy in guessing peoples sexual orientation from their facial features. This might be slightly higher if you can also see their clothes and gait. However, statistically, you would have to be impossibly good to be most accurate when guessing anything else than "they're all straight" (because statistically and presuming the science is somewhat accurate, most people are straight and if you guess that certain people are not you're more likely to be wrong). :-) So yeah, you can guess, but it's not accurate.

Apart from buttons and rainbow flags I don't think there are any specific clues or vibes we give off. Some conform to the stereotypes, most people don't. I suspect those who conform especially much are actually trying to, consciously or not. These people are often not very popular with other gay people, as we're generally not fond of our stereotypes. I haven't been with other gay people besides my boyfriend for a long time though, maybe I'm inaccurate too.