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. 
It is also like Schrodinger's Cat in the sense that hopefully whatever is inside is not still alive. But seriously: the main thing is time. You start with the recommendations from your manufacturer (since all cookers vary somewhat in terms of pressure and therefore temperature) and then keep good notes as to whether you prefer slightly more or less time. For example, I've learned that with my cooker, if I'm making pinto beans without soaking, for whole beans I like about 35 minutes, but if I'm going to puree them, 40 is better. 