I would also suggest you see if there is a local dedicated pinball forum where you live or else you could also join pinballbash.com. You might not know any local pinball collectors that could guide you so I will give you a few tips that were passed onto me.
Before you go look at the machine, head on over to ipdb.org and download the PDF manual and take a quick read through it. You might even want to print off certain pages in case the seller doesn't have the manual for the machine. The one page I suggest you print off is the switch locations page, I will use Demolition Man as an example.
When you go see the machine make sure it is not on already and if it is simply flip the switch on the bottom right off and then turn it back on after 2 seconds. If the machine starts up and doesn't beep or display "Please press ENTER for TEST REPORT" you should be good. Sometimes you can get this error but the pin will still function and play fine as I mentioned earlier if the machine hasn't been played in a while or a particular switch has been engaged the machine will report an error. If you get this message open the coin door and there should be a 4 button service panel on the coin door. Here is a pictue from my Flintstones and you can see this service panel.
Hit the ENTER button and pay attention to the DMD as it will list off any errors the machines detect. You might have to write some of it down and if you happen to miss one just hit the EXIT button and hit ENTER again to re-run the TEST REPORT.
I will give you an example from machine, you might get a message that says "CHECK R.RAMP EXIT SW47 ERROR". This means the very right ramp exit switch #47 might be a problem. I removed the glass and located the switch and checked the wire form and pushed it forward like the ball would to see if it engages the switch. See this image here so you can follow what I am trying to explain.
Simple solutions to this problem could be:
a) wire form needs to be adjusted so it engages the switch
b) switch is sticky
c) wires are not attached to the leads on the switch or possibly a break in the wire connecting to the PCB
As you are doing this you will want to be in the TEST MENU -> SINGLE SWITCH TEST.
To navigate it is pretty simple as I believe the two middle buttons are labelled as "+" and "-" and you can already guess + advances the menu option and - goes back to the previous option. Once you have the TEST MENU option up hit enter and it will take you to another sub menu where again you will use the + and - to cycle through the different options.
After you select the SINGLE SWITCH TEST the DMD will display a mapping of different switch locations and when you engage it a block will appear on the DMD and disappear when you disengage it. There are other types of switches as well such as target switches and you can easily test these by just pressing on them with your finger and there are roll over switches such as the ones you see on the out lanes and again just press down on them with your finger to test.
The next test you will want to do is the GENERAL ILLUMINATION TEST. If you notice a lot of lights out this could mean:
a) bulb needs to be replaced
b) bad bulb socket from corrosion
c) break in the wiring
Usually GI lighting is connected in series so when there is a serious problem an entire be string could be affected. If you are new at this I would probably walk away from the cab unless it is priced just right cause it could end up costing you more money in the end and you would have been better off just finding a better example of the same machine for a bit more money. Let's face it you want to spend more time playing your new "old" pinball then repairing it
There is one more test you can do is the FLASHER TEST. Keep in mind the newer machines have a safety feature due to HIGH VOLTAGE and when the coin door is open the circuit is open and the FLASHERS will not go off. You can either locate the white pegs the coin door triggers and push them in with your fingers or just close the coin door and you will see the flasher go off. I find a good test to run is under the UTILITIES MENU called AUTO BURN-IN. Run it and close the coin door and look at all the inserts, pop bumpers, GI, etc.
Going back to my initial point of turning the machine off and then on. You might not see the TEST REPORT error message but you might notice that a message "Factory Settings Restored" appear. This could be a good or bad thing. It is good if the seller say he just put in fresh batteries prior to you arriving and this message is normal whenever you change the batteries. At this point you are best to open the back box instead of taking the seller's word and inspect the battery holder and look for any signs of corrosion like this:
The problems I've seen from battery corrosion is it is not localized and can do quite a bit of damage on the underside of the PCB and even on IC chips adjacent to it! Now if you don't see any corrosion I would switch the machine off and turn it back on to see if the message appears again. If it does there is probably corrosion damage that is not visible without removing the board to inspect the other side. Thank the seller for the opportunity and walk away cause you saved yourself a lot of money and back ache cause these damn machines are heavy... ask my brother in-law and he'll gladly flip you the bird lol
I hope I'm not scaring the OP away but better to be informed of these things then to spend his hard earned money and be stuck with a huge paperweight. You might also want to read over pinballninja.com which is an excellent blog site which documents tons of common repairs on many different machines ranging from Gotlieb, Stern, Williams, Bally, Sega, Data East, etc.
-mike