If memory serves, I recall Bretons tend to have dark hair while Imperials tend to have dirty blond/light brown hair.
Bretons can have with Blond or Light hair. For example, Lazare Milvan and Norbert Lelles, Biene Amelion, Antoinetta Marie, etc.
I don't think there is any way to tell from the faces. They are shorter than Imperials, particularly the women.
The male voices are certainly different, though the female voices use the same voice actor.
The best way to tell for named NPCs is the name. Bretons tend to have french-ish sounding names, imperials more ancient roman-ish sounding names.