At military airfields, most SPs (now called Security Forces can act as customs) are customs/INS qualified to inspect the plane and take our forms.
For example, on returning from OIF, I was a cargo courier on C-141, and we stopped at Pease ANGB, in NH
, our port of entry, where the SPs took care of our forms, and inspected some bags, etc. usually not a big hassle. I've stopped at Bangor, Maine, a big one for us in AWACS, and many other bases on the east coast.
Stopping along the way, each country has its own rules, when we stop in England they just do a check of how many folsk are on board, Ireland will not let us out of the terminal, etc, etc.
Here at Tinker we have a customs guy, though he is a pain in the ass. Old civilian who opens everybag, and cubby etc, and can take hours on the airplane :-( he is notified as we are inbound, and his sole purpose is to work our Customs stuff.
Ciao, and Hook 'em Horns,
Lt-AWACS, Yankee Air Pirate