A kid asked, “If a Pope is a bad Pope, but he’s still the Pope and there’s nothing we can do about it, how do we know when we can ignore him?”
Short answer: when he opens his mouth.
Long Pedantic Dad answer: whenever he says something a) novel, or b) contrary to what the Church teaches, or c) factually errant, or d) contrary to common sense.
P.S. No disrespect to the Orthodox (or Protestants, Jews, Muslims, Atheists, etc), but ideas have consequences, and either you’re right, or I’m right, or we’re both wrong, and pretending otherwise is false charity.