I would say experience. Like hunting, there is no set way to find what you're hunting, you gotta learn.

Use reads/stats on people. E.G. A guy has never raised you ever before in the last 18 hours you've been playing with him, and all of a sudden he is raising and 4 betting you on this hand, obviously he is going to be at the strong end of his range that you have him on.

Stuff like that, I'm not that great at it, but that is my basic knowledge on the subject.