Quoted from Coyote:Assuming you are meaning aside from the usual inspection points for a WPC machine (power board, MPU, cabinet, etc..)
I would check the operation of the machine - magnets, clock, gumball machine. Check for the powerball. Any playfield damage around the scoops, rocket kicker, etc.
Test the two powerball detectors (gumball and trough). The trough one is getting hard to find.