Being gay can not determined by mere mannerisms. In other words not every feminine acting male is gay and not every gay male acts feminine. If you really believe that he might be gay then maybe slyly ask him what turns him on and make subtle convesation to find out about some previous sexual partners if you don't already know. You could also honestly ask him if he finds other men attractive but only if you believe that he won't be offended.

Hope this helps.

—2cents