I can't really put my finger on it. After watching both species for so many years, quite often at great distance where the individual details like ear size or the animal's color don't stand out, I can just tell what I'm looking at.
The way they disperse in a group, the way they walk and move, where they are in relation to the terrain and available cover. I don't know.
I'm no Jim Bridger but I pretty well always know what I'm looking at.