Charles Spurgeon Commentary John 1:32-33

"And John bare witness, saying, I have beheld the Spirit descending as a dove out of heaven; and it abode upon him. And I knew him not: but he that sent me to baptize in water, he said unto me, Upon whomsoever thou shalt see the Spirit descending, and abiding upon him, the same is he that baptizeth in the Holy Spirit." — John 1:32-33 (ASV)

And John bare record, saying, I saw the Spirit descending from heaven like a dove and it abode upon him. And I knew him not: but he that sent me to baptize with water, the same said unto me.

Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.

I do not doubt that John had assuredly guessed that Jesus was the person; but he had nothing to do with guesses: he was a witness for God, and he could only speak as God revealed things to him.