I have always explained:
Magi, and other hedge wizards being imperfect creatures, not perfectly aligned with the magic realm, have minute flaws in their casting spells that leave minor, identifying spell "residue" (aka casting sigil).
The question now arises - do magical creatures leave "spell sigils" when using their powers?
I would have thought "no", as they are aligned with the magic realm. But they are not necessarily perfect, are they? So now I am undecided.
Good question! We see, that magi, being the rare intelligent social animals among spellcasting beings and hence more often casting under observation by other spellcasters, have such sigils and tend to have them known among sodales.
If magus Conradus Filius Laurentii would observe a group of magical greylag geese closely over some time, would he distinguish identifying characteristics of their powers?
But would anybody have known about them before Conradus' groundbreaking tractatus Hic sum! Ubi es? were distributed in the last Folio?
the question came up in my game with the troupe fighting a group of magical giants with the odd magical power (they chant ancient runes that our Danish speaker barely recognises)
I would say yes, though it would be difficult. The mages can identify other mage's sigil, but a similar spell cast by a dragon would have mostly the "dragon flavour" only one truly expert in the matter would then be able to distinguish one dragon's sigil from another. Similarly, I'd expect the dragons to find it easier to recognise each other's sigils, but would take more exposure to hermetic magic to tell one hermetic sigil from another.
props for writing what I assume is a joke... in latin. 