If you can or cannot see sustained spells have been a topic we discussed a few times already and we never really came to any conclusions as far as I can recall.
We are in agreement that active magic (such as a sustained or quickened spell, an activated focus, a preparation, a spirit, a dual natured entity, a wholly astral entity etc) is typically immediately obvious and does not require a test for an astral observer (there are a few exceptions, for example sustaining spells while you have the extended masking meta magic).
But SR5 is not very clear on what you can and cannot see from the physical plane.
SR5 mention that most magic is obvious and does not require a test (for example indirect combat spells 'throwing fire from his fingertips' SR5 p. 281 being the subject of manipulation spells such as Control Actions or the glowing effect of the Armor spell SR5 p. 292).
It also mentions that 'any form of magic (conjuring, spellcasting, enchanting, magical lodges, spirits, etc.) changes the world around it' (on the physical plane).
Subtle magic we know for sure that you can spot with a regular perception test (because they are explicit rules or examples) are:
- An astral form (spirit or projecting magician etc) passing through your aura - chill or tingling sensation, SR5 p. 314
- Stepping through the astral form of a ward - notice the markings or feel the tingle, SR5 p. 281
- A magician performing (casting - yes; sustaining - maybe, maybe not) a spell - twitchy fingers, SR5 p. 280
- Being the subject of a subtle manipulation spell (such as control thoughts) - feeling of dread, SR5 p. 292
Book also mention 'shimmering air' caused by spirits on the astral plane (but book does not present any details here - perhaps they are talking about the ghostly image of manifesting spirits or projecting magicians, perhaps they are talking about adepts with the detect magic adept power that let them sense spirits close by without using astral perception, perhaps anyone can at any time take a matrix perception test with a threshold equal to 6 - force of the spirit to notice the shimmering air it leaves on the physical plane).
This list is to be seen more as examples and is probably not exclusive, but we know for sure that the list above are things you for sure are allowed to notice with a regular perception test. There might or might not be
more things you can spot beyond what is listed above (but not less).
SR6 is also a bit unclear on what you can and cannot notice, but it seem as if actually noticing magic is a bit toned down in this edition and you are now looking for things that does not really belong and things that seem to be out of the ordinary rather than tingling sensations, feeling of chill or dread etc. Noticing 'twitchy fingers' when a magician is casting a spell is still explicitly mentioned, but not so much beyond that...