A couple quick points to ponder:
Check the sticky about the function test, which enables you to test the trigger group and safety.
Examine the rear peep sight to see if it sits tightly in its base, the peep is centered and it adjusts smoothly.
Visually examine the bolt fit into the receiver. The bolt locking lugs on both sides should contact snugly with the receiver (look at it with light from below).
Cycle the action to check for smoothness of operation. It will work better if properly lubed (I think a sticky explains this).
With the action removed from the stock, check the alignment of the op rod and piston. The guide through which the op rod travels may be loose, but can easily be fixed.
Check for barrel timing. If the upper hand guards contact the stock on one side, or the front sight looks canted when viewed from the rear, that could indicate off timing. A bit's okay and it can be fixed, but it's better to have a properly timed/aligned barrel, especially if you plan to use the open sights.
Since it came up with the SOCOMs, you might also want to check to see that mags easily insert and come out of the rifle.
I'm sure that there are other factors to look at and that others more versed in these rifles will add their tips, but hope this provides a decent start at what to look for.
