Elder D. Todd Christofferson of the Quorum of the Twelve Apostles has taught that “repentance means striving to change” and that “real change may require repeated attempts.” He has also said that “for our turning to the Lord to be complete, it must include nothing less than a covenant of obedience to Him,” which is contained in the baptismal covenant and the sacrament (Oct. 2011 general conference). Full repentance also means making up for any harm you did to others. In addition, the Lord has said that a person who fully repents of his sins “will confess them and forsake them” (D&C 58:43). You should confess all sins to Heavenly Father and serious sins to your bishop as well. (When in doubt, talk to the bishop. He’s there to help.)
After doing these things, a way to know you’ve fully repented is to see and feel the effects of repentance—changes in your desires, feelings, outlook, relationships, and behavior. And, most importantly, full repentance will bring the companionship of the Holy Ghost.