How can I tell if the food inside a pressure cooker is done cooking? I've been looking into getting a pressure cooker since it works on the premise of increasing the boiling temp of water thereby cooking things more quickly. Operating on the premise that a pressure cooker is an enclosed system, is there a way besides following the time recommendations of a recipe to find out if it's "done", or is it like Schrödinger's Cat where you only know for sure when you open it and release the pressure.
Smell.

Every time the whistle of a pressure cooker blows, you get a mild aroma of the food along with the escaping steam. The aroma is usually a good indicator of how good the food has been cooked. 

Besides smell, the only other thing is time.