Albert Barnes on John 2:25

Should testify of man. Should give him the character of any man.

He knew what was in man. This he did because he had made all Jn 1:3, and because he was God, Jn 1:1. There can be no higher evidence than this that he was omniscient, and was therefore divine. To search the heart is the prerogative of God alone (Jer 17:10); and as Jesus knew what was in these disciples, and as it is expressly said that he knew what was in man--that is, in all men-- so it follows that he must be equal with God. As he knows all, he is acquainted with the false pretentions and professions of hypocrites. None can deceive him. He also knows the wants and desires of all his real friends. He hears their groans, he sees their sighs, he counts their tears, and in the day of need will come to their relief.