Spellcasting steathily

Sorcerers have 'subtle' metamagic to steathily cast spells.
Non-sorcerers may still attempt subtly with their spells. A bonus action can be spent to attempt to avoid detection with a spell.
Somantic (S) components
Stealth DC = 15 + target's Insight(WIS) bonus.

Verbal (V) components
Performance DC = 15 + target's Perception(hearing) bonus.
DC +5 if target is 'expecting' to see a spell being cast.

Roll both checks for spells with both components
Failure on either roll results in the spell being apparent.
Success results in the target not knowing the spell is in the process of being cast. Results of the spell may make the casting of the spell obvious after the casting is complete.