You can learn about profiling, That would help. If you can sense auras or vibes as I call them, that can also be of help.
I assume that it is next to impossible to know in exact and precise detail what a person is thinking at any given time. Taking circumstances in consideration and things such as personality traits, beliefs, gestures, etc., one can indeed have a good idea of what someone else might be thinking at a specific time.
This is not something easy to do.