Well, by your description of her/him, she/he sounds like they're either a tomboy, or doing their best at being male even though they can't come out for some reason. If you want to ask them, you should get to know them better. Learn her/his name, what nickname they like, etc. before you ask. That alone might help you decide if asking them outright is the right thing to do, or if she/he would be offended.
If you do decide to ask her/him, don't just say "Are you a transsexual?", say something before to at least warn them that you're not trying to be disrespectful. Maybe like "I want to ask you a question and I'm sorry if it offends you...". If you plan to be friends with her/him or want to help her/him with transitioning if they are trans, then maybe you should consider telling her/him first that you're trans, and see how they react. That might soften the question, so that it makes sense why you would notice something like that.
Whatever you choose to do, consider her/his feelings, and make sure that you're not going to make them mad or offended by asking. And of course, don't ask where everyone can hear. Good luck!