Actually, if you're going to get the glass off, you might as well just use the testing utilities to check the head. Test #14 is the Head Cycle Test. It will rotate the head clockwise, kicking the coils where they should be kicking, then go backwards and do the same thing. This will repeat until you stop the test.
If the head doesn't go back and forth, seems confused about the proper rotation, or kicks the coils on the wrong faces, you have a problem with one of the parts I mentioned above. If you're even a bit savvy you can fix these parts pretty easily, and use the malfunction to try to drive down the price.
Of course, the seller may just tell you to piss off, at which point it becomes obvious that he'd rather sell the machine to someone unaware of the problems with the machine, thus making him untrustworthly.