Grant (or maybe it was Wes) once wrote up a terrific guide to checking upper receiver dimensional trueness. That was back around the time that Mark came out with his Stealth upper and the boards when crazy with debate.
For what it's worth I never found a CMT that was true on its face -- out of a sample numbering something under 12. Every Sabre upper I've purchased has been dimensionally right-on-the-money -- out of about 5.
I realize this doesn't answer your question about how to go about the actual checking but I think I'll let the guys with the big credientials speak to that point.
Best of luck.