Basically for a 2 or 3 roller mill you want the rollers to be clean, free from rust/corrosion and still have a decent amount of knurl to them (the rough teeth), and the rollers should move easily. The adjustment screws sould be straight and hold the roller in place and not be crossthreaded or have messed up the threading otherwise. Basically a well maintained roller mill will look similar to a new one with only cosmetic damage and some small wear to the rollers.
Then again this is my limited knowledge of mills. Mine looks new still, but I have only pushed a couple hundred pounds of grain through it.