arcane rainbow vellum

If an arcane caster casts a nondamaging arcane spell on the vellum, the vellum will generate a multicolored blotch resembling a Rorschach test.  The spread of colors identifies which schools of magic the caster knows and the relative size of the different colored parts provides a rough estimate as to how powerful the caster is in each school.   Black for necromancy, red for enchantment, orange for invocation, blue for abjuration, green for transmutation, yellow for divination, purple for conjuration, and shifting colors for illusion.   The vellum will not reveal which spells the caster has specifically though.   The vellum is very brittle so if a caster does not work measuring a mage who only knows invocation since invocation spells will obliterate the vellum.   The vellum is less informative with divine magic cast at the vellum always makes a silver blotch regardless of what divine spheres the caster has or who their divine patron is though stronger casters tend to create larger blotches.