hmm you probably just have to make stuff up, and you have to do that anyways at some point.
Knowledge Arcane has some pretty much out-of-reach ideas
and concepts, that Commoners have no chance of understanding.
Even something with a DC of 11, is completely off limits to most Townspeople.
Wizard says Expelion-potato-gnosis, and the people just go
aDMg says Expelion-potato-gnosis and the people go: Ooohhhh that's what he was talking about.
Comprehend Languages has some tangential info to this.
Un-Comprehend Languages, is a game historic idea,
either as a named Counterspell, or even useful itself.
"Spellcraft + Read Magic" combos are listed in Spellcraft.
This implies things not just
above normal usage, but also below.
Somatic components can be tracing symbols as part of a casting.
Real Life example are Chinese who speak too divergent dialects
tracing out shapes of Chinese writing in the air, to communicate.
It works. Sigils can be the same idea + usage.
Complete Arcane + Mage books , have piles of fluff in them too.
You might find something in them, not exactly hard coded rules.