Poo will look a lot like bird poo, only runnier. It'll have a darker part and a white/yellow part to it. Sometimes depending on the size of snake and whether its eating adult mice or whatnot, you may or may not find hair in it. Poop smells like poop.
A regurge looks like a mouse that's partially rotted and will smell like it. You'll typically be able to discern features of the mouse..feet, eyes, tail, etc. The smell is unmistakable.
Most snakes housed on a loose substrate typically defecate around the edges or underneath the substrate. So you might poke around the cage and see if you can find a dried brownish/black with powdery white/yellow glob.