Spectral, a trigger is attached to COMBAT_RESULTS_TRIGGER, then refer to the documentation for:
GetSavedCombatArmyHero()
GetSavedCombatArmyCreaturesCount()
GetSavedCombatArmyCreatureInfo()

Example:

COMBAT_WINNER = 1;
COMBAT_LOSER = 0;

Trigger(COMBAT_RESULTS_TRIGGER, 'CombatResult');

function CombatResult(id)

  local n_stacks, creature, count, died;

  -- analysis of the losing side
  n_stacks = GetSavedCombatArmyCreaturesCount(id, COMBAT_LOSER);
  for i = 0,(n_stacks-1) do
    creature, count, died = GetSavedCombatArmyCreatureInfo(id, COMBAT_LOSER, i);
  end

  -- analysis of the winning side
  n_stacks = GetSavedCombatArmyCreaturesCount(id, COMBAT_WINNER);
  for i = 0,(n_stacks-1) do
    creature, count, died = GetSavedCombatArmyCreatureInfo(id, COMBAT_WINNER, i);
  end

end