SUE - If you have committed adultery before you were baptized into the LDS church does your baptism wipe away this sin even if you didn't confess it to anyone? And even though you may have been a member of the LDS Church for many years and have ask Heavenly Father for forgiveness many times for this before and after baptism, is there something else you need to do to get forgiveness if your being baptized wasn't enough? And how do you know if you have been forgiven or not?

JOEL - Before baptism the convert will have been taught the gospel, including the law of chastity and the doctrine of repentence, and it is assumed that before baptism they will have repented of any sins against chastity and will have promised God to never do it again.
In the pre-baptism interview they will simply be asked if they understand and accept the doctrines of the church and beleive that have repented of their past sins.
Although confession is good for the soul, before baptism a new convert is not yet under the jurisdiction of any earthly church authority and therefore is not required to confess sins to anyone in the church before baptism.
In the repentence process one must however of course confess to anyone hurt by the sin and to God;

I, the Lord, forgive sins unto those who confess their sins before me and ask forgiveness (D. & C. 64:7.)

They must also demonstrate godly sorrow for having committed the sin (2 Cor 7:10); promise God they will never do it again; and commit themselves to living a righteous life. God said:

"he that repents and does the commandments of the Lord shall be forgiven;" (D&C 1:32)
"Behold, he who has repented of his sins, the same is forgiven, and I, the Lord, remember them no more. By this ye may know if a man repenteth of his sins—behold, he will confess them and forsake them." (D&C 58:42-43)

The moment one comes out of the waters of baptism their souls are considered clean from all previous sins committed before the baptism(Acts 2:38, Acts 22: 16, 1 Pet 3:20-21), including adultery; which means they should also consider themselves forgiven for those sins. When one is washed clean they realize they are forgiven when a feeling of peace and comfort comes to their souls. Mosiah records:

"And it came to pass that after they had spoken these words the Spirit of the Lord came upon them, and they were filled with joy, having received a remission of their sins, and having peace of conscience, because of the exceeding faith which they had in Jesus Christ who should come, according to the words which king Benjamin had spoken unto them." (Mosiah 4:3)

One proves their repentence and assures forgiveness by living a faithful life in the gospel from then on. God said:

"All those who humble themselves before God, and desire to be baptized, and come forth with broken hearts and contrite spirits, and witness before the church that they have truly repented of all their sins, and are willing to take upon them the name of Jesus Christ, having a determination to serve him to the end, and truly manifest by their works that they have received of the Spirit of Christ unto the remission of their sins, shall be received by baptism into his church." (D&C 20:37)

The bottom line is, for a new convert, repentence and baptism are enough for remission and forgiveness of all sins. Anything a person did before baptism is between them and God. Any guilt that one still feels for past sins before baptism, can be discussed with their Bishop, who can help them figure out why they don't yet feel the peace of forgiveness. Any serious sins a member commits after baptism must be confessed to the Bishop.