OK, so I'm going to do this thing. I wanted something like it in my setting anyway. The idea will be built around these two core ideas:
Residuum. The lingering traces left by spell casting. These can take many forms, but are usually some combination of smells, sounds, sights, tastes, emotions and other sense data. Different kinds of spells have different flavours of residuum. Residuum varies in intensity depending on the power of the spell that left it.
Another sort of residuum are the traces left by certain extra-dimensional beings. Some monsters, like demons and elementals, are not native to our plane and their presence leaves behind physical trace evidence (for example, demons might leave traces of sulphur behind them, as they do in the show Supernatural).
Imagos. The magical fingerprint left by every spellcaster when they cast a spell. Like residuum, this will generally be some unique combination of sounds, sights, smells, emotions, and other sense data. The imagos of a spellcaster may develop over their career, but will always be identifiable to someone familiar with it.