Originally Posted by
Unregistered
By the number of +'s in the skill description; they range from 0 to 2 +'s, respectively.
Prefix-wise...
Fire - Burning = M, Inferno = L
Water - Blizzard = M, Cocytus = L
Wind - Storm = M, Turbulence = L
Thunder - Plasma = M, Impulse = L
Dark - Abyss = M, Schwartz = L