The answer to this question was in my daily devotional reading today. The quote is from the writings of J.F. Clark (1810-1888), an American theologian and writer.
“We need to know that our sins are forgiven. And how shall we know this? By feeling that we have peace with God,–by feeling that we are able so to trust in the divine compassion and infinite tenderness of our Father, as to arise and go to Him, whenever we have sinned and say at once to Him, ‘Father, I have sinned, forgive me.’ To know that we are forgiven, it is only necessary to look at our Father’s love till it sinks into our heart, to open our soul to Him till He shall pour His love into it; to wait on Him till we find peace, till our conscience no longer torments us, till we can feel that our sins, great as they are, cannot keep us away from our Heavenly Father.”
P.S. Lutherans also know they are forgiven when they receive the absolution every week as part of the liturgy. What a blessing! As Martin Luther said about the Gospel– We need to hear it every week because we forget it every week.