An alligator jumper is your friend. Classic circuit, 4 reels must be on zero and their zero position switch closed for a part of a reset seq to be complete. All the reels are on zero yet the motor runs and runs. Solution, take an alligator jumper and clip it on one end of the chain and move it down switch to switch until the db relay pulls in, you found your dirty switch...
Or BankABall, 7 daisy chained switches on the relays in the head, which is the dirty one preventing the special?
Or game remains in tilt, on of three slam switches really not closed? which one? Jumper them.
Last night my Surfer would hold in the tilt hold, ran a jumper from 30v to the relay as a work around while I fixed something else. Then I noticed later same slam switch circuitry had a run to the coin lock out relay...and what do most of us do? disconnect the coin relay on the door because they are always burnt. And it happened to be a "loop thru" (not the end of the circuit but two wires on 1 lug), my visual inspection showed the previous repair dude forgot to twist the two together leaving my slam circuit opened. Fixed.
And lets say you want to verify all the bulbs of the match circuit, either find the stepper and advance it by hand in game over or use a jumper and run down the bulbs...
Also, one thing to note, the "state" of the cir diagram is ball one, ball in shooter lane waiting to be plunged...
Ken