In addition to mof's list, even before purchasing, I:
1. move every single solenoid plunger by hand, to do a physical check, making a note of the wear on a plunger, or examining the bakelite for cracks in an early SS.
2. look at the flipper mechanisms closely. Check the switches, check the plunger, link, and pawl, check the nylon bushing.
3. Check all the tilt switches. Sometimes they are cut. This can indicate...things.

When I get it home, I always:
1. Replace every coil sleeve
2. Re-pin every plug (I don't, but now have the tool, so I WILL, and you should too!), unless this is obvious been done
3. Clean that playfield! And then wax it. But really, clean it, clean it, clean it!
4. Checking every fuse cannot be stressed enough.