It used to be that while walking somewhere, the guy who looked back at you at the same time you looked back at him was a good candidate to be gay. But that's cruising, a different subject.
Seriously, with some guys I just knew. Other times, like when I went to Dignity/Washington in DC in the '80s and '90s, I'd see hundreds of gay men of different ages, ethnic backgrounds, walks of life (although skewed to white-collar government workers, priests or others of religious orders, and college students), etc., etc. I learned there wasn't really a single
aspect of appearance I could go by but more the countenance and manner, the look in the eye. I imagine my gaydar was also poor unless I saw men in a gay setting or we actually interacted -- whether verbally, non-verbally, physically...
There's been some pushback on the study from GLAAD and HRC, but here's a piece arguing they misunderstand the science:
HRC and GLAAD release a silly statement about the ‘gay face’ study from The Bilerico Report on LGBTQNation. The authors of the study addressed the controversy, too, and in a very readable form that's accessible and understandable to lay people.
I recommend it, if you have a few minutes to spare.
I agree it's a phenomenon worth studying. Heck, with the new iPhone X coming out, maybe Apple will know if you're gay!
The tech is out there, to paraphrase The X-Files, and if the research becomes reproducible and them implemented in a reliable manner -- sounds distant, but things change quickly these days -- it could be used by homophobic governments or groups to target gays.
~ Bob S.