Same way you know know you have food in your stomach after you had your breakfast.

I invited Him in and in that instant I felt his presence in me, from that time my life changed completely and for the better. I have done and seen miracles and I have never felt this clean (spiritually) and free as ever before.

Meeting Father God and uniting with him through the salvation of Jesus and the indwelling of the Holy Spirit is a extremely noticeable and life changing event.