US lens & gray market Nikon lens--Question re: service/identify

The question I have is how are you able to tell, identify, if one's lens (Nikon) is either US or gray market lens as I have failed to keep my paper work at the time of my purchase. I would like to have my lens serviced by Nikon, however, they will not service the gray market lens. I could just send in the lens & await their response, however, I may want to send in an additional lenses for routine maintenance. I hope this makes sense to you & thank you for your response.

Or: do you recommend another source to have lenses, cleaned, focus checked, maintenance et al other than Nikon and not caring about US or gray?