I don't understand the question. Are you doubting John's testimony? He tells you explicitly about the signal.
Let us have a look at Luke 3:21-22,
Now when all the people were baptized, Jesus also was baptized, and while He was praying, heaven was opened, and the Holy Spirit descended upon Him in bodily form like a dove, and a voice came from heaven: “You are My beloved Son, in You I am well pleased.”
I take note of the fact that Jesus' baptism took place while a crowd of people watched. The dove landing on Jesus was a sign for John, but the voice from heave was verbal confirmation from the Father.
In John's first letter he mentions that the voice of the Father confirmed the sonship of Jesus audibly three times during his ministry.
1John 5:5-9 -- speaking about God's testimony John writes.
Who is the one who overcomes the world, but the one who believes that Jesus is the Son of God? This is the One who came by water and blood, Jesus Christ; not with the water only, but with the water and with the blood. It is the Spirit who testifies, because the Spirit is the truth. For there are three that testify: the Spirit and the water and the blood; and the three are in agreement. If we receive the testimony of people, the testimony of God is greater; for the testimony of God is this, that He has testified concerning His Son.
Three times, the Father spoke from heaven saying "this is my beloved son."
The Spirit = At the baptism of Jesus
The Water = (cloud) At the transfiguration
The blood = At the resurrection (See Romans 1:4
People don't believe words. They need to be shown.