Only God has heart knowledge

There is a special Greek word for heart knowledge: kardiognostes. It comes from two other words:

Kardia = heart
Ginosko = knowledge

It's only used two times in the New Testament, and both times of God.

And they prayed and said, You, O Lord, who know the hearts of all, show which of these two You have chosen (Acts 1:24)

So God, who knows the heart, acknowledged them by giving them the Holy Spirit, just as He did to us (Acts 15:8)