The proof is based on whether or not the person in question has received a witness that God lives and that the scriptures are true.
Once that is done, the scriptures (along with personal revelation and other sources) confirm those statements.
God created Christ:
Acts 2:36
Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.
Christ is identical to God:
2 Cor 2:4
In whom the god of this world hath blinded the minds of them which believe not, lest the flight of the glorious gospel of Christ, who is the image of God, should shine unto them.
John 5:19
Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise.
Christ has a physical body
Luke 24:39
Behold my hands and my feet, that it is I myself: handle me, and see; for a spirit hath not flesh and bones, as ye see me have.
God has physical body
Acts 7:55-56
But he [Stephen], being full of the Holy Ghost, looked up stedfastly into heaven, and saw the glory of God, and Jesus standing on the right hand of God,
56 And said, Behold, I see the heavens opened, and the Son of man standing on the right hand of God.