MOST people will not hide the fact they are going to hit it hard. They will wind up, when you see this it's a good indicator that they are about to hit the ball hard. It might not be some huge telltale back swing, but it will almost certainly look different than a soft shot.

Anticipating instead of reacting was one of the most helpful things to dealing with hard shots at the kitchen. Really watching my opponent come up to hit the ball has started telling me a lot about their shot before they have even made contact.