I'm not certain if there is a fool-proof way to confirm a legit USA body.
When considering buying used, I ask where the body was originally purchased and whether the seller still has the original receipt. If so, I make sure that it's an authorized dealer on Nikon's USA web site. Lacking that, I want to know if the equipment has been serviced by Nikon USA, and if so, when.
Lacking that paperwork, I like to call Nikon with the serial number, explain that I'm considering buying this used item and ask for assurance that it can be serviced by Nikon in the USA. And/or ask if the item has a service history with them.
They seem to be reluctant to use the term "grey market" in my experience, but once I explain that I've been victimized in the past (purchased a brand NEW body from a non-authorized dealer that as it turned out, had been on a pallet stolen off of a shipping dock, and was subsequently seized as evidence) - Nikon generally confirms whether the used item can be serviced.