In truth nothing is foolproof. I've been to places that give you ammo, tag your RIF etc but there would be nothing stopping me turning up an NPAS or similar in the field. I played first time round about 20 years ago, then got back into it recently, and I've never been spot-checked in the field. The only thing I've ever seen that was lairy was at DogTag last summer: a guy chronoed as a DMR (he was right in front of me in the queue), then in the field I clocked he was firing full-auto. I told a marshall, they spoke to him and reported back to me that he had turned off the semi-lock on his mosfet. They sent him home instantly.
I do some marshalling at a site in the SE when I'm free and often run the chrono. Do the same checks in the chrono, tag the RIFS (and regs for HPA) then eyeball everyone out of the safezone to make sure everything is marked up. When I go out into the gaming area, I take a chrono out in my pocket and I have spot-checked people. I've yet to come across anyone doing anything silly like adjusting power post-chrono.