To start, download the manual from the White's website and read through the "quick start" section to learn how to get started. Then, you can do a bench "air test". Turn the machine on and go through the menu and find a program, coin or beach as they have the icons turned on. Get it set to hunt. Be sure the coil is hanging off of the counter so that you don't have interference from the metal frame of the countertop glass. Don't try to "ground balance" inside the store. You can test the machine without ground balance. If they'll let you take it outside to test it, you can go through ground balancing the unit and then test the unit with coins on the ground.
Take some pocket change and wave a coin in front of the coil to be sure it gives a good tone and that the screen gives you a VDI number and the correct coin icon shows up. If the shop owner has any silver coins, ask if you can wave one of them by the coil too to test it as well. Vary the distance from the coil to the coin and note the information on the screen and the sounds. The further away the coin is from the coil, the sound should be different and the info on the screen will change as well.
If you buy, be sure that the detector includes a rechargable battery pack & charger, a second battery pack for 8-AA batteries and manual.
If the AA battery pack is missing, one can be purchased from White's for around $10.
If the rechagable battery pack & charger is missing, you can purchase those as well for $50 & $40 respectfully.
The manual can be downloaded from the White's web site.
Reason I'm telling you this if for haggling purposes with the pawnshop.
REMEMBER, whatever they are trying to sell it for, they have MAYBE half that in it... You can get it for less than asking price, and if anything is missing use it for negotiations.
If you find after you buy it that something doesn't seem right about it, White's has EXCELLENT customer service, you can send it in and let them check it out and service it if needed.
Good luck and let us know...