Stupid question: What's the ultimate difference between a mammalian molar and premolar? I can tell the difference between them in a given skull, the premolars are usually smaller and simpler (with exceptions of course), but are there any universal diagnostic features?

I don't think there would be any generalization among all mammalian orders and anything specific in shape certainly wouldn't apply to some extinct early mammalian orders which had lesser differentiated teeth. The only thing in common is probably tooth position. Some orders have bicuspid premolars but that's probably not universal.