Ask him to spend time alone with you.
If he disagrees, he is probably not interested in having more than a superficial relationship at this time. Back off and give him space. If he agrees, he’s either interested in a closer friendship or may be interested in you romantically. Flirt with him, such as telling him that he is your perfect physical type, and see if he reciprocates.
Express your feelings through your body language.
Look her in the eyes when talking, smile and see if she reciprocates. If she turns away from you, scans the room while you are talking or crosses her arms, she’s probably not interested in you at that moment.
Use touch, selectively.
If you are getting good signals from the flirting and his body language suggests he may be interested in you, reach out and touch his hands while you are talking or press your hand against the small of his back while you are walking. If he does not pull away, he is interested in you. If he pulls away, do not show that you are embarrassed; after all you took a risk and sometimes these do not pan out.
Ask her a hypothetical question and see how she responds.
You can say something like: “If we were together, how would you have handled that?” If she responds by discussing how she handled the situation with an ex-boyfriend or her current boyfriend, she does not see you two together as a couple.