Which ending is the Ture ending and which one is the alternative good ending
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By observing Third Guardian's dialogues after the combo attack.
Spoiler below.
When the Third Guardian reply "I'm amused, mortals. Very much so." and then "How are you guys able to come up with such a great coordinated attack?" -- This is clear sign that you've landed on the TRUE Ending. It boosts the love meter of every individual that are involved in this battle.
Otherwise, the Third Guardian's set of dialogues - "I'm not amused." and then "How are you guys able to come up with such uncoordinated attack, and yet managed to survive?" -- This implies that he is greatly disappointed. Regardless, you get to continue the story with negative points to all individuals involved in this battle. This is Alternative Good Ending.