You know how to conceal your spellcasting.
Benefit: You may make a Spellcraft check to conceal the act of spellcasting from opponents observing you. Your check is opposed by the Perception check of each observer. (If the spell has a casting time of more than 1 full round, observers may make Perception checks once a round to notice your spellcasting.) If you attempt to conceal a spell you must use a material component or a focus to cast, you suffer a –5 penalty to your Spellcraft check. If you attempt to conceal a spell with no verbal component (including spells using the Silent Spell feat), you gain a +5 bonus to your Spellcraft check.
Casting a spell secretly takes at least a full round action, even if the spell’s normal casting time is less than that. If the spell has no listed visual or other sensory effects, any opponent who fails the opposed Perception check is unaware a spell has been cast (although they may deduce magic is in use if they observe the effects of the spell).
If the spell has any listed sensory effect (which includes any spell that deals damage), observers become aware you have cast the spell once you are finished, even if they fail their Perception check.
Adventurer’s Handbook: Genius Guide Volume 1