Inspecting a game before you buy it

Steps

Take off the playfield glass, start a game and work every switch, rollover, target, slingshot and bumper by hand to see that each one scores or fires.
Remove the ball, lift the playfield and prop it on its support bar, then look underneath for water damage, rodent nests, chewed or loose wires, missing parts and amateur rewiring.
Look for burned or seized coils on dead flippers or bumpers; they are cheap to replace and can be a bargaining point.
On a solid-state game, look at the batteries and boards in the backbox for leaked-battery corrosion, and at the board connectors for scorching or discoloured plastic.
Treat heavy rust on coil plungers and greasy, sticky switches as many hours of cleaning ahead.

Source: Pinball Machine Care & Maintenance, Bernard B. Kamoroff (Bell Springs Pinball Alley)