Aside from checking all switches (lots of optos), flashers you want to make sure the Mini-Playfield is working well. Also, the trapdoor off the left side of the big ramp. WPC games have issues with resets and there's well documented repairs for that.
Mini-Playfield: lots to go wrong with that. In test, you should be able to drive it up and down. All the mushroom buttons work. Dalek Flaps engage and release (inside the chutes of the MPF). Cracked plastic dome on the MPF. Locks/lock ejects.
Playfield: Slings, playfield wear on ball drop, playfield wear coming out of the mini-playfield. Does it have lightning flippers or regular 3" bats (stock is lightning flips).
Ramps: Especially the front edge of the right ramp. Who-Mobile covers.
Topper: Plastic dome over the top and mini-Dalek in the dome w/eye stalk. These are unobtainum and rarely are on the market. Possibly there s a moving Dalek motor in the topper. Test 15 will allow you to put it through the paces.
Cab: the ink fades, especially the red.