Good question. No person has seen God at any time (John 1:18), but the One being in the bosom of the Father, He has made Him known. (ibid) As to your honest question, I use a reverse line of sight. The Genesis account says we are made in His image. So we must share that image in common. Take a Picasso painting,... How do we know it's a real Picasso? I suppose we can never be 100% sure...