Two options :

• UnitAffectingCombat("unit") lets us check the combat status of a unit. This would be put in Unit test. However it may be incorrect, as there are references to it not returning the right value on proximity-based aggro until the creature attacks or is attacked.
• InCombatLockdown() lets us check if the player is in combat (through restriction of the API). This would be put in Character, as we can't check this for units.
  We can do both if it makes sense.