How can we be assured of this pardon?
God's Spirit witnesses with our spirit that we are children of God.
Catechism for the Global Methodist Church, question 60
15For you did not receive a spirit of slavery again toward fear, but you received a spirit of adoption as son, in which we cry, "Abba, Father." 16The Spirit himself testifies together with our spirit that we are children of God. 17And if children, then also heirs, heirs of God and co-heirs of Christ, if indeed we suffer with him so that we may also be glorified with him.
26For you are all sons of God through faith in Christ Jesus.
6And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" 7So you are no longer a slave, but a son; and if a son, then an heir through God.
13In him you also, after hearing the word of truth, the gospel of your salvation, and after trusting in him, were sealed with the Holy Spirit of promise. 14Which is the guarantee of our inheritance until the redemption of the acquired possession, to the praise of his glory.
7For God did not give us a spirit of timidity, but a spirit of power, love, and self-discipline.
15And the Holy Spirit also bears witness to us; for after saying, 16"This is the covenant that I will make with them after those days, declares the Lord: I will put my laws on their hearts, and write them on their minds," 17then he adds, "I will remember their sins and their lawless deeds no more."
Assurance set the early Methodists apart, and the answer to this question is a sentence of Scripture. ¶102 of the Book of Doctrines and Discipline says that pardon "ordinarily results in the direct assurance of 'God's Spirit witnessing with our spirit that we are children of God' (Rom. 8:16) as well as the indirect witness of a good conscience in the midst of the fruit of the Spirit." A forgiven person can know that he is forgiven, because God's Spirit tells him so.
Romans 8 describes the direct witness. (Romans 8:15-17) The order runs from sonship to the Spirit's cry: "because you are sons, God has sent the Spirit of his Son into our hearts, crying, 'Abba! Father!'" (Galatians 4:6). Believers "were sealed with the Holy Spirit of promise, which is the guarantee of our inheritance" (Ephesians 1:13-14).
The ground of this confidence lies outside the believer. "You are all sons of God through faith in Christ Jesus" (Galatians 3:26), and the Spirit bears witness to that fact. In Hebrews the Spirit bears witness through the new covenant promise, "I will remember their sins and their lawless deeds no more" (Hebrews 10:17). The assurance is an assurance of pardon already granted, so Paul can tell Timothy, (2 Timothy 1:7) Wesley distinguished the faith of a servant, which fears God and does right, from the faith of a son, which knows itself forgiven, and the church's standard says the second is the ordinary result of justification.
For a Methodist a settled sense of being God's child is an expected gift. ¶102 says "ordinarily," and Wesley allowed for exceptions. A believer who lacks the feeling of assurance is pointed to the promise and to the fruit of the Spirit in a good conscience, and is told to keep using the means of grace where the Spirit does his work.