Simple Q&A by RAW for Pathfinder 4

[Psyren]

Q 367

If a spell is being cast without any components (due to it being Silent and/or Still), can others identify it via Spellcraft?

A367: Official answer is yes, - componentless magic (including SLAs, Psychic Spells, and Still+Silent+Eschew arcane spells) can be identified as long as you can perceive the caster in some way. Unfortunately, the only rationale behind this appears to be "it just works, okay?" or maybe there's some kind of light show that accompanies spellcasting that the rules neglected to mention until now, but there you go.