Strangely, my only talent when telling people of different ethnicities apart are telling the major European groups apart. I can tell a Spaniard from a Frenchman from a German-- but don't ask me to tell a Brit from a German. I blame it on lots of exposure to French people in highschool.

Granted, it only works on natives, so it might be mostly cultural cues that I'm picking up on, but I can think of at least three visual cues for a French person off the top of my head and how to differentiate them from the Spanish.