The core event here is UNIT_SPELLCAST_SUCCEEDED (hereafter _SUCCEEDED). This event fires whenever a spellcast is completed server end, regardless of whether it is instant or cast. Therefore whenever an instant cast has been used successfully, this event will fire. However, there are times when _SUCCEEDED will fire but the spell is not instant. The events UNIT_SPELLCAST_START, UNIT_SPELLCAST_FAILED, and UNIT_SPELLCAST_INTERRUPTED (hereafter _START, _FAILED, and _INTERRUPTED) allow us to track precisely which spellcasts are instant and which are not.