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I learning that Italians don't like phrases like "I know," without something that indicates what it is that one knows. Still, it is funny when I imagine me saying "io so," and my interlocutor standing there thinking, "He knows... the combination to his gym locker, How many jellybeans fit in a shoebox, That a dogs mouth is cleaner than a humans???" And me responding to his quizzical expression with "IT, Pasquale... I know IT!!"
I believe we don't really have two different ways to say "I know" and "I know it". You'd just say "lo so" in both cases and that would be correct. I guess the voice tone would play its role in emphasizing whether you know IT or you just know.
Saying just "so" would sound really weird and unnatural.