The bolt CARRIER has NO bearing on headspacing.  Headspacing occurs between the barrel extension (which is, for most purposes, a permanent part of the barrel) and the bolt.  Headspacing can be measured using ONLY the barrel assembly and bolt, without an upper, carrier, or anything else.

-Troy

Buy a FIELD guage from Bushmaster, which is made to M16 (5.56mm) specs instead of the much-more-common bolt-rifle (.223) guage.  The M16 guage is slightly longer, allowing a bit more headspace, which is necessary for a military-style semi-auto.

If the bolt won't close on the field guage, you're good to go.  Test once a year or so.

If the bolt DOES close, then you need to buy and try a new bolt.  If the bolt STILL closes, you need a new barrel.  With the AR, there's really no way to "adjust" headspace except to swap parts, except for when the barrel is being manufactured.  Short headspace can be corrected if the rifle is NOT chrome-lined, but long headspace means a new barrel.

Fortunately, it is FAR more likely that you'll wear your barrel out long before headspace becomes a problem.  But if you keep your rifle long enough to go through a couple of barrels, your bolt may get worn enough that it will close on the Field guage, and you'd need to replace the bolt.

-Troy