This is one of the biggest questions I get, and the answer is... you just will. Parents worry so much about how they'll know if the baby is happy or sick or whatever else. Dads especially are so nervous — they assume the mom will just know, that the instinct is delivered with the placenta or something.
It never ceases to amaze me what these same parents say when they come in at the one month visit. They tell me exactly what their baby likes, what it doesn't like, how it likes to be held, and so on. It just cracks me up, how this "helpless” baby that can't even talk is so completely able to communicate exactly what it wants. Somehow, it just works.