What do you do when you are not sure if a girl you like likes you if she has made out with your best friend and your best friend likes her?

Never go out with your friend's partners, even if they only dated for an hour. If you like that much, then find out how your best friend will feel about you asking this girl out. Dont end up with out a friend for a curiosity. It's always good to compete, but if it is at the expense of a good friend, then it's best not to stress it. But if you converse with your friend on the situation, then he should understand. If you both are at an agreement, then competition should ensue. Besides, that's the "natural" way of obtaining a mate. Furthermore, if envy comes forth, then he's a friend that can't respect your infatuation with that individual.