Unless it's a model that was only offered in Europe, like a 420SL, then deciphering the radiator plate is the only reliable way. People bringing euro cars into the US will replace bumpers, speedometers, etc. to the US version and people with US cars will decide that they like the Euro headlights, bumpers, etc.
About the only "Euro" features that I don't see people fawning over are the cloth seats and the km-only speedometers.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino