While you shouldn't be rude or overly sexual, nor should you say something to the person you're talking about, try commenting on how attractive someone of the person's gender is, as well as someone of another gender, and see if they agree or seem fascinated.
7 Signs Your Boyfriend Is Secretly Gay: How to Tell If He's Hiding His True Self
This is used to provide traffic data and reports to the authors of articles on the HubPages Service. Recognize that bisexuality is not the only option. It's probably not sexual. These are covered in detail in the book. My longtime friend and colleague Dr. How do I know if a girl is bisexual?