observe their body language.
when the girl opens up, let's say palms up legs not crossed. smiles, leans to you. holds your hand, allows you to hold her hand. (shaking hands is definitely suggestive)
simple things like that means she's open to you. but when her legs are crossed, or hugging herself, such things means she'll give you a hard time.
rubbing her shoulders, or arms means she's a sensual person. or when she loves laughing at jokes, she's fun.
you'll pick things like this through experience.
observe and learn. most importantly dare, to try it in order to weed out the things that does not work.