How do you tell when a bunny is hungry?

Whenever the food bowl is low and/or if you walk by him/her and they suddenly perk up at the smell of food. But the latter is not always reliable; s/he may just love that treat and want it even if s/he is not hungry.

You don't. You give it food whenever it is out of food. Always make sure your rabbit's water bottle is always filled, too. Your rabbit will like you better if you give it extra special care.