You could have a measuring plug made up, with an external pipe thread on one end, and a center drilled recess, or a punch mark on the other end. Don't forget wrenching flats so you can get it out.
... but that's gross overkill, really, and it's still awkward to use with a caliper, unless you add a sliding square leg, but that's really overkill.
A framing square and a tape measure should be sufficient for any reasonable purpose except manufacturer inspection.
Mike Halloran
Pembroke Pines, FL, USA