At night- when you meet a chick you want to bang, lightly touch her before you speak or as soon as possible thereafter.
During the day- when you meet a chick you want to bang, at least lightly touch her sometime before you ask for her phone number. If she's not busy, don't go for the phone number- try to take her somewhere.
This communicates your sexual interest in her and also screens for her sexual availability.
If she touches you back- it's a green light. If she does nothing but lets you touch her- it's a green light.
If she backs off, but doesn't leave- it's a yellow light.