where I could take it to have someone check whether it's 8mm or if it's been rechambered?
The world is a pretty big place.
I would recommend going to Google maps, find your house, then search for gunsmiths.
No idea what rifle you are talking about, but if it left the factory as 8mm, and has the factory barrel, then it is still 8mm. There is a small possibility that it was re-chambered to 8mm-06, pretty easy to tell by trying to chamber a .30-06 case. If the bolt won't close, it is most likely still 8X57.