"Detection spells: These spells enhance the senses." I don't think you can simply notice someone using a Detection spell. You might use Astral Perception and try Assensing but that's it.
Street Grimoire, page 12: "Magic is normally an invisible process to the non-Awakened. A normal can no more discern the casting of a spell than a casual shopper can read the coding behind a sales algorithm directed against them." and a couple of spells tend to describe "invisible" when dealing with Mana spells. You can interfere that most mana spells are invisible and Detection spells are invisible.
Perception usually needs to beat a treshold or another test. You would roll to see the caster, not the spell.
Which is the rules contradicting themselves again, as magic warps reality in ways that can be detected from the meat world, as noticing the casting is seeing the unnatural ripples in the world.
What, if anything, can be seen of active spell effects is dubious. Personally, I find completely undetectable magic that only affects the values of the numbers you're throwing at each other boring and flavorless.
Invisibility is one thing. Undetectable is its point. But an armor spell even less detectable than an invisible force field? Lame.
At a low force armor spell, you shake someone's hand and they'll probably notice. Medium force and you may notice dust, debris, and rain not touching them in their protective bubble. High force? Frag subtlety. You're in a magic glowing suit of armor.
Boosting someone's strength? A small boost would be subtle, but if you go whole hog shooting someone up into the outright supermetahuman, they're gonna hulk out.
Case by case, omae.
And those detection spells are some of the most detectable to the trained eye. You don't even need to spot the magic. Humans are used to interacting with humans with the same basic senses. You can generally tell when someone is focusing on any given sense. Sight, hearing, smell. Unless you actively hide noticing something (an opposed skill roll), you have tells. For magical supersenses, you still have physical tells, which are liable to be bizarre and alien compared to those with normal human senses.