Here's a list of items to look for in Glocktalk's archives.
http://glocktalk.com/forums/showthread.php?t=1155960
Here is the summation posted in #13
-note: #6 isn't going to happen unless they let you test fire it on a range.
1. Visual inspection of exterior for damage, scars, wear.
2. Look for hogged out pin holes in the frame. Press them out and put them back to make sure everything is correct.
3. Check the breach face. If it has a inward dimple around the firing pin hole, this will indicate the pistol has been shot alot, maybe with hot hand loads.
4. Check recoil spring by aiming gun verticle (unloaded), no magazine, pull slide back and slowly let forward to see how slide goes into full battery.
5. Load mags for function.
6. Cycle loaded mag through gun by racking slide.
7. Dry fire for function. With the trigger held back, push on the top of the chamber. If there is a lot of movement, it's pretty worn. If there is just a little, less so, but keep in mind, even the new guns will have a little play when you do this.
8. Field strip
9. Inspect internals for cleanliness, wear.
10. Check for aftermarket internals.
11. Run your pinched fingers down the barrel feeling for bulges.
12. Look for any rings or pitting IN the barrel.
13. Make sure the extractor is not broken
14.Check documentation of test fire date, S/N, matching numbers