Arcane Sight and detecting Silent Still Spells/Spell-like Abilities

[Zorgoth]

Do you think that Arcane Sight gives the ability to tell that a spell or spell-like ability with no verbal or somatic components is being cast (by detecting a new aura)? Obviously it can't identify the precise spell (since at most it lets you know the school of an aura), but would you register a new aura during the cast, or only after the spell is done being cast? This could be relevant if you want to ready an action to try to interrupt the concentration of the caster of a still silent spell or spell-like ability with damage.