Watch the M14 Forum. A Springfield Pre-Ban model was just on there for $1700. In order to verify that they're GI parts you'll need the serial numbers. Important parts to watch for is the serial number on the receiver heel, the bolt, and the op rod mainly. If you get really picky you can watch for the trigger and sights as well.

The manufactures for the US GI M14 were Springfield Armory (SA), Winchester, Harrington and Richardson (HR), and Thompson-Ramo-Wooldridge Inc (TRW). Springfield are the most common and the most manufactured. While Winchesters are rare and bring a lot higher priced. TRW on the other hand is very valuable and demanded as they exceeded the requirements on the specs and their parts are know to last the longest in opinion.

Numbers to watch for to verify it is a GI part:
GI bolt: 7790186
GI op rod: 7267064
Trigger group: Manufacture Stamp: SA, TRW, HR, 7267030
Hammer: 5546008
Gas cylinder: 7790902

On GI manufactures parts, there will also be an initial of the manufacture on the part such as SA, TRW, H&R, or HRT. There is a wealth of information on the M14/M1A platform out there and I'm just starting to learn about them after about 10 years with the weapons. I've forgot more than I remember hence I have the books for fast reference.

Lee Emerson makes some great books that are worth purchasing for reference, "M14 Rifle History and Development." Along with The M14 Owners Guide and match conditioning instructions" by Scott A. Duff and John M. Miller and the shop manual from Jerry Kuhnhausen.

Getting a rifle as close to USGI as possible is hard but very feasible if you take your time. To me, the biggest things I'd look for is the serial number on the receiver to verify it was built during the proper time, a USGI bolt, USGI op rod, and preferably a USGI gas cylinder. Everything else is fine. If you don't get a USGI bolt, it's easy to swap out the ejector and extractor with USGI parts to get them up to spec. GI op rods are very pricey at around $250 and up but the current ones being manufactured aren't bad. Just my opinion on them for the amount most people shoot and problems that have been discussed.

The USGI parts numbers listed are from "M14 Rifle History and Development" on page 144.