So I was thinking of creating a homebrew 1st level spell called detect power level, which would detect a character's power-level based on the targets HD/level. So the question is how should the numbers scale and should different classes/HD have different multipliers to their HD to judge their power level?
It would probably be easier just to have it reveal a creature's CR, rather than having to calculate another number for you to have to calculate.
However, if you want a number that's not CR exactly, maybe just make a chart where Power Level = CR x N, where N is a fixed number you choose at random.
Detect Evil/Good/Law/Choas already has this (too an extent) with the strength of the aura's present. Just add in a component for true neutral (Detect Neutral) creature and you can even get an OVERWHELMING neutral aura!
The strength of the aura tells you the relative HD of the creature based on some additional knowledge and can even stun you if its overwhelmingly strong in comparison to you.
Its over 9000! ...
Sorry I couldn't resist.