Just play it. All players... Pretty straight forward.

Open it up an lift up the playfield
The wiring harnesses are tight , if somebody has messed withe the game you will notice
Or you will notice extra wires that are plastic coated

Agree with above. You can generally just tell when something isn't factory. The biggest test though will be to play through ALL players and make sure the game starts new games correctly with reset targets and scores.

I suppose if one wanted to really check that *everything* is working, it could be a rather long list. But something like:

- remove the playfield glass
- make sure the score reels are set to non-zero values on all players (unless it's a woodrail, without score reels)
- make sure there are zero credits
- plug in and power on the game. If the game lacks an on/off switch (and many older ones do), then it requires a coin to be dropped in
- add coins to see if credits are added
- press the start button to see if the restart sequence begins
- check that all the score reels correctly reset to zero
- check that the credit unit decrements one per player
- check that the first ball is kicked to the plunger
- if a multi-player game, then press the start button to add all the players; check that the number of players lights light up
- check all the playfield lights
- open the backbox and check all the backbox lights (this may be challenging, as some games require the backglass to be removed, others the backbox board can be tipped back).
- check that all the flippers work, can be held on, have good strength
- check all rollver and standup targets manually. Make sure all of them register and score the correct number of points
- check all the pop bumpers, slingshot kickers, and dead bumpers for action and proper scoring
- check all other playfield scoring features present (drop targets, vari-targets, spinners, discs - whatever can be hit and score points)
- check that all the score reels are incrementing properly, and that the next score reels advance when the previous rolls over from 9
- drain the ball
- check that any bonus points are correctly tallied
- check that the next ball count correctly increments, or that the next Player advances
- if a multi-player, check that the scoring is now registering for the correct player
- drain the ball and repeat for all players
- tally all the high score values and see that replays are awarded (for replay games), or that extra balls are awarded (for add-a-ball games) (and use this to check that the knocker is working)
- complete any playfield objectives and see that any Specials are lit; collect the Specials and see that replays are awarded
- check that the proper chimes or bells are operating, associated with the correct point value
- drain the ball until past ball 5 for the last player, and see that the game goes to Game Over
- see that a match light is lit (if lucky, see if a replay is awarded on a match)
- repeat from the top, but only as far as seeing if Tilt is correctly detected and operates
- if the game supports a 5-ball or 3-ball setting, repeat draining all the balls on each setting to see if Game Over is detected in both modes (if care)
- after manually testing everything, then play a few real games and see if the game is operating in a responsive manner (can keep up with all the random scoring, seems snappy, plays fast, etc.)

Hmm, that's all that comes to mind offhand. I'm sure I've missed stuff.

But that's just the functional portion. There's also the cosmetic, and then the subjective "yes, it works, but how *well* does it work" stuff.

Good luck!

Sort of off topic but I accidentally discovered several unknowns, or wondering about, about my game, 4 Player GTB Jungle, by going to YouTube and lucked up and found a great video, by Corey Cooke, playing a game like mine and I could see what works right, how it works, exactly how and what and when lights work, what switches control each if the three chimes which mine were wired wrong and I had to just switch them around a little.

Hence a simple, somewhat boring video, of a game being played has a wealth of knowledge on the little seemingly insignificant, but also important things, about a game that you won't really know until you pay close attention, IE the chimes thing on mine.

Coreys video told me a lot about my game. No Light Box shot but what he has here worked great for me

I'm new here and don't know if this site has such a feature but if there is a sister site, YouTube?, that all perfectly working pins had a good video of the game played through all five balls showing how it is suppose to be,, that would be a godsend for us newbies to refer to before buying a game or even seeing if our old games are in perfect working order after the buy. Even after years of having a game maybe.
I might have gone years and not know that my chimes were wired wrong, or even thought about it.

So before I buy another game YouTube will be one of my first places top check out.