Let's take this for an example: My Imperial thief wants to marry another Imperial so he doesn't feel like a wierdass. In a bar, he finds 2 young girls in their early 20s, an Imperial and a Breton.
Now unless Bethesda makes Imperials' voices strictly American and Bretons' voices strictly French (which 11 times out of 10 they won't), there is no way to tell.