The odd mii out is either :
A) Looking the opposite direction as everyone else (if everyone's head is turned to the left, it's head it turned to the right)
B) Walking differently than that rest of the mii's. This one is really tough to notice, usually the odd one out is stepping it's left foot forward while everyone else is stepping their right foot forward and vice versa. It's really subtle, but the difference is also reflected in the arms swinging differently.
Hope this helps, I got gold at it and haven't touched it since, not my fav. on WiiPlay, but I had to at least get gold, lol.