How do I know if a MC is genuine?
good question. (it appears Butch added alot of info while i'm previewing my post!)
first off the PMC is very easy to determine by the color inlays, knobs, pickups and PMC swirl. See the spec sheet and photo gallery. It's hard to dupe the PMC neck since no other guitar had that fretboard inlays. Use the info above to rule out fakes rather easily.
the GMCs are trickier, since it is more plausible to take a JEM777VBK and turn it into a "GMC" by getting a replacement body, two-color pickups (or leaving them green and calling it a "prototype" ) and a new ATD (or even a Herc swirl).
You could inspect the neck heel joint for the ATD stamp (not always there) and again examine the guitar's attributes, history and signs of part replacement (ie. new pickups are a dead giveaway). Re-soldered switches, etc.
The neck plate serial numbers are not trackable and essentially worthless. They could point out obvious problems (ie 88 neck plate in a GMC) but could be easily changed/swapped to match the expected date/serial# for the guitar.
Darren/ATD's swirls are "authentic" only on the original GMC, PMC, MC pieces. Use common sense and post photos here before buying if the authenticity is a concern. Good luck... glen