I don't know, I wish many people would answer this.
God knew for sure, that this was His Son. Christ lived as a human lives while He was here. We are told in 1 Peter 3 that He died in the flesh and was made alive in the spirit. But surely Christ was also God when He was in the flesh. Otherwise Christ could not have lived a sinless life.
1 Peter 3:18 For Christ also died for sins once for all, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit;