Currently, CLE(U) has two events for when something dies: UNIT_DIED and PARTY_KILL.
UNIT_DIED contains info about *what* died (name/guid). It doesn't say *who* killed it or whether or not it was tapped by the player or their group.
PARTY_KILL only fires for kills made by the players *party* (sub-group in raids). It doesn't fire if someone outside of the player's sub-group kills something in a raid.
How are we supposed to detect raid kills? A RAID_KILL event should be added (or GROUP_KILL, following Blizzard's recent tendency to add *Group* API functions), firing for kills made by other raid members (meaning the mob is tapped by the current group).
Currently I see no way that is 100% failsafe to check if a killed mob was tapped by the player or group.