I John 5:12: He that hath the Son hath life; and he that hath not the Son of God hath not life.
You know if you have the Son of God because this "witness is in yourself".
I John 5:10: He that believeth on the Son of God hath the witness in himself:
I John 5:20: And we know that the Son of God is come, and hath given us an understanding, that we may know him that is true, and we are in him that is true, even in his Son Jesus Christ. This is the true God, and eternal life.