Well there is no certain way to find out if someone is bisexual or gay unless you ask them. Yes it is a tough thing to do but it is the easiest way to figure out what their sexual preference is. If you do get the courage to ask them remember that some people maybe not out to the public about it and will not appreciate if you ask them. It is tough asking someone because people get offence about it, or will feel frightened by the question, and this may result in anger.
There are other ways of ‘beating around the bush’ and finding out if they are bisexual or gay. You could ask them how many girls they have dated or questions to see if they answer it like a straight man would or if they answer it differently. Asking them who they like…
You could watch them and see their different body positioning and actions to see if they look a bit homosexual. The way they walk, talk, etc.
You could also just talk to them more and gain their trust. if you feel ready you can tell them that you are bi and he might support you or/and even tell you if he is or not.
There are many ways you could find out if they are gay or bisexual, the easiest way would be asking them… even though there is nothing wrong with being gay or bisexual, it is still offensive to some people. Just make the right choice and do what you feel is right! It is a bit of a tricky question…
Hoped this help a bit!