Does anyone know how to know if your friend is gay/bisexual?

*They are never in a relationship with someone of the opposite sex.
*They never make references to people of the opposite sex in terms of attraction. Alternatively, they may make such references 'all the time', as a sort of cover.
*See how the person reacts to LGBT topics. On the one hand, they may be extremely committed to gay rights, or on the other hand, may openly voice homophobic comments. This can be quite common among people who are not yet 'out'.
*You get the vibe that they are attracted to you/are flirting with you.
*Check their social media profiles. Even if they are in the closet, you may find clues as to their sexual orientation in the things they post, or the friends they have.

What you shouldn't do is make assumptions based on someone's looks, style, their choice in hobbies or entertainment, or how they talk. These are all silly myths, and there is no direct correlation.

Coming out is a very good sign.

If they were taking it up their bunghole