You can distinguish them by looking at the amount of "waves" and "ticks":
no ticks (grey-green) = level 1-2
Two tick (green-blue) = level 3-4
Four ticks (blue) = level 5
Six ticks (blue-purple) = level 6-7
Six ticks, 2 of them connected (purple)= level 8
six ticks, all connected (purple) = level 9
six ticks connected, 2 half-waves (purple) = level 10
six ticks connected, 2 waves (purple-gold) = level 11-12
It might be difficult to distinguish 1 with 2 and 3 with 4 (but really, the difference in fact is minimal, you won't need to distinguish them really). The most difficult is 6-7, that might be a problem, but still the time you'll be playing with 6-7 gear characters is very limited.
To help you, gear level is written in Roman numbers under the character rendering. You can also enable, since Android Lollipop, color correction in developer settings.