API UnitIsTappedByPlayer

Returns whether a unit is tappe by the player. isTapped = UnitIsTappedByPlayer("unit")

Arguments

 * unit : String (unitId) - unit to query, e.g. "target"

Returns

 * isTapped: Flag - 1 if the unit is tapped by current player, nil otherwise.

Example
You can use this function in conjunction with and  in order to check whether the player will receive any benefit from killing the unit: if UnitIsTapped("target") and not (UnitIsTappedByPlayer("target") or UnitIsTappedByAllThreatList("target")) then -- You'll get no quest credit/loot if you kill this unit end