One way to tell, is to sit very close to him. When you like someone, your personal space doesn't apply to them. This means that they won't move away if you are inside their space. (this does hold true, I am a guy)
Just tell them
you should propably tell you best friend that is a girl she can help.
If your best friend doesn't know you like your crush than tell her how you feel about it. what you can also do is tell your friend that you liked him first or tell him to decide between you two. or you can also share him. or you or your friend can move on.
If you tell your friend then she would say ( WHAT ) If you don't tell your friend and your date knows that your friend have a crush on him, he might tell your friend that he has a girlfriend already.
if there is a chance of your friend being with their crush back off if there is not or only a small chance or their crush likes you then tell your friend i told my best friend she was fine with it i mean its only a crush its not like they are actually dating
BE HONEST. It sucks,( for your friend) and it awkward for you but in the end it's better to tell your best friend. Maybe the crush has a friend for your besty
You don't.
flirt with her then be bestfriends, then tell her!... to be her best friend act gay! ahha jk
You tell your friend you saw what she did and see where it goes from there.
Tell him i don't think this is right, tell the truth.
When he tells your best friend, that he have's a crush on you.
Tell him ''Be nice or I'm done with you!''