I'm not sure there's much description of how active spells 'look' in the lore, it's possible that after the initial 'impact' it becomes a tiny little speck of Aqshy in the brain, and you'd need to be Volans to notice it.
Edit: or maybe even Volans couldn't notice it, camouflaged as it would be against natural Aqshy in the brain; the whole think kinda depends on what boney rules the spell to look like as it works.
According to the Roleplay book, the presence of an active magic spell is pretty obvious to anyone with magical sixth sense within a few yards - it's the most basic and obvious function of magical senses, much easier than seeing the Winds themselves. Just being in the presence of someone with an active spell should be enough to tip off a spellcaster, even without looking at them with Magesight. Identifying what the spell does is harder, but the simple presence of a spell is obvious.
There are ways of hiding magic, but I doubt the Bright College are experts at it. By default, Warhammer magic is the opposite of subtle.
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