Things to look for in a good AR...
1. Is the gas key staked
2. If it has an adjustable M4 style stock, is the castle nut staked
3. Are the bolt carrier group, chamber, and barrel chrome lined
4. Has the bolt been magnetic particle tested (it will have an "MP" engraved on it if so
5. Does it have good upper/lower fit, can it be disassembled/assembled easily by hand
6. Does it pass a basic function test a.pull the bolt back and chocolate chip cookie the weapon b.place the weapon on safe and pull the trigger, it shouldn't fire c. Place the weapon on fire, pull the trigger, the hammer should fall d. without releasing the trigger, recharge the weapon, release the trigger, and listen for reset (you will feel and hear a soft, distinct click)
If it passes all these little tests, it is probably good to go. If not, I would be leary and look at something else. In my opinion, the best AR out there for the money right now is the Smith & Wesson M&P15.