Questions: Flirting

Like are you aware enough to notice signs they definitely are into you? or are you mostly oblivious unless they just tell you?

Say I'm dancing with a girl in the club and I find her really attractive and I want to let her know that, which of these words would you like to hear the most?

And say I manage to give her that compliment and she just says thanks instead of "you're (...) too!", which she might not do for whatever reason, would it be a good idea to just ask her point blank "do you find me (...) too?" It would just make everything so much easier, and I wouldn't be wasting my time on someone who isn't attracted to me.

PLEASE ELABORATE ON YOUR OPINIONS

This always seems to happen in particular with Spanish and Latin American girls named Carla, hence being coined the Carla situation:

You meet an incredibly attractive girl named Carla at some social gathering. You both hit it off immediately and there is clearly mutual attraction and flirting. You get her contact information and start chatting. However, the very next time you meet up with her and her friends she is flirting with another guy at the club/party and ends up going home with him. What do you do?