Your best indicator is smelling the powder. Don't worry so much about the expiration date. If your powder is old, it will have a fowl smell. Thats in indication that the can is left open or opened alot becasue oxygen breaks it down.
How long are you going to keep it? Even a big tub is only 80 servings which should at most last 3 months. If you're really worried you could probably put half of it in a Ziplock and freeze it.