As you already own a 580EX II you should be pretty familiar with it.
First set the power to max and fire off a couple of pops with the test button (take fresh charged batteries with you for this). Make sure it charges quickly & silently, obviously if it doesn't fire there's a problem.
Make sure the zoom function works by manually changing it, you should hear the motor & see it move if you look through the lens.
Check the wide angle diffuser and bounce card are both there and that they come out and go back smoothly, the diffuser should cause the zoom to move. Watch for the spring at the back being in one piece they can & do break.
Attach the flash to the camera in ETTL mode and check it reads the info properly, fire off the odd shot to check it does communicate correctly and then the histogram to make sure it's not wildly out.
Other than that the only other thing is the physical condition which should be obvious, other than the battery compartment for battery leakage.
I think that just about covers everything, no doubt if I missed something someone will point it out, although I think that most flashes will be in pretty good order.