I think ‘guess’ show that you have some clues but not all, so you are not sure While ‘assume’ means you take something as a truth without any proof.

Guess is if you don’t have clue at all, so you guess. Assume is based of something, for example he just eat so I assume he is not hungry

I would say assume :)

“I guess he’s busy..”This is the most common phrase in the U.S.

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