Clean with a low round count, they sell for around $1200 used.
The real tell tale sign is going to be removing the action from the stock. If the glass bedding is still in goodshape, it will take great force to remove the action from the stock. If the receiver just drops out of the stock, then the glass is worn and you may have to have it rebedded (read will open up the groups and you will have flyers).
Note: you only want to remove the action from the stock maybe once a year or less,or you will ruin the glass bedding due to pull wear.
Since you thinking of buying it, a good sign will be that the piston, the guide rod, recoill spring and the opt rod are on the filthy side when you pull the action. This means that the owner was not pulling the action out of the stock to clean the inner parts at the same time as the rest of the rifle. Also, chances are the the owner never pulled the piston (looks for marks on the gas plug), if so and you have the tools, you can get a real good idea of what ammo and how many rounds was fired threw the weapon (signs of miltary primer sealant, and overall fouling in the piston/cylinder). As for using a throat gauge, don't even try. The throats are reamed less than zero on USGI gauge for a NM or Douglas Super match barrel. You can mic the muzzle, but then again, you will need to pull the FS to do so (may not be a bad idea just encase he distroyed the muzzle crown by jaming bushed down the wrond end).
To add: To tell a nm from a loaded model, the stock will be glass bedded, and the front cyclinder/hand guard will be unitized (look at the back of the hand guard band for two screws just above the pistol channel/behind the hand guard). The rear sight will be hooded, but this could have been added to a loaded model sometime down the road.