The only way to know if someone is gay is to ask. Or better yet, focus on your own life, and give the other person some privacy.
If you mean how can you tell, you can't just by looking in most cases.
you can't spot a gay man you can flirt and see how he reacts
or you can go to a gay bar
Sometimes, for some gay guys they don't like receiving anal sex. In most cases though, gay men can and often do ejaculate when receiving anal. The prostate is a g-spot so to speak for men and will make them ejaculate when stimulated.
Gay men are already men.
Gay men are individuals who are attracted to other men romantically and/or sexually. Sexual orientation (being gay) does not influence someone's gender identity (being a man). Men, whether straight or gay, identify as male based on their self-perception and expression.
When women are gay, they are interested in other women. When men are gay, they are intersted in other men
Only straight men hate being called gay. Most gay men prefer the term "gay."
Gay men usually chase other men.
About 3.5% to 8% of men are gay. That is a small minority.
There is no odor particular to gay men.
Gay men do not dislike lesbians.
Gay men wear the same kinds of underwear as straight men.
Some gay men do, and some don't.
About 3.5% to 8% of Swedish men are gay.