Someone asked us recently the following question. "How does one know that Hashem has accepted his Teshuva, especially for sexual sins?"
1) Chaza"l say that true Teshuvah is acheived "Ki'sheme'id alav yode'ah talumos shelo yashuv lekislo od" -Translation: "When G-d, who knows all hidden things, bears witness on him that he will not go back to his bad ways again". But how does a person know when G-d bears witness on him? The Ba'al Hasulam explains that when a person truly feels that it would be as bad for him to go back to his old ways as sticking his hand into fire, that is a sure sign that Hashem has testified on him that he will never go back to his old ways!
2) The Ramba"m writes that true Teshuvah is achieved when you have the same opportunity to sin as you did before, and the same desire, and yet you don't. If that happens, you will know that you have done true Teshuvah.
3) The holy Chassidic master, Rabbi Mendel of Vitebsk writes (in his sefer Pri Ha'aretz) that true Teshuvah is through Messiras Nefesh (to be ready to die for it) and he explains that a person can reach this level if they feel so bad about their sins that they would rather to be dead than to do these sins again. If a person feels this way, then they know they have done an exalted Teshuvah.
May Hashem help us all achieve a true Teshuvah!