Whenever you cast a spell or activate a spell trigger or spell completion magic item, you can attempt a Bluff skill check opposed by a Sense Motive check from anyone that can see you. Those who fail the opposed check don't know that you cast a spell or activated an item unless it has an obvious effect that emanates from you. In addition, the magical auras of magical effects on you and magic items you carry (except for artifacts) can't be detected.
