How would one spellcaster determine the level of another? Reasons would include things like, can caster X cast Fireball, is this caster a better match for Caster Y (level 3) or Caster z (level 16), etc.
It is up to you to put the flavor on it that you want, but I am certain there is some in-game terminology for spell levels, and caster levels.
Every caster would know that an apprentice, no matter how gifted, wouldn't be capable of a meteor swarm, and that even darkvision is beyond most apprentices.
Beyond that, it would depend on if the "target" caster would display their proficiency.
Your world might refer to "circles of power" that match the level of spell - fireball would require mastery over the third circle of power.
You could use a color code to represent the levels, and resistor codes covers that pretty well:
Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet = 7
Grey = 8
White = 9
Or whatever you would like. If there isn't a symbol or color the target caster is wearing, I don't think there is any way another can measure the power... Not without researching what the caster has been known to cast.
Well, I was wondering about something similar to Detect Magic's Aura Power levels. I did manage to find what I wanted - Arcane Sight. It gives the caster the ability to determine the strength of the most powerful spell the target currently has available for use. This will suffice for my purposes.