I want to know how I can ask about past relationships of my lover.
I assume that by "lover" you mean the one you love and not that you are having sex outside of marriage. If you are sexually involved, you are already off on the wrong path. See: Why Sex Outside of Marriage is Wrong.
There is really only one question that is important to know about a person's past and that is whether they are eligible for marriage. If a person has been married before and divorced for any reason other than because their spouse had committed fornication, then that person is not eligible for a second marriage. Beyond that, you should be looking at who a person is and not who they once were. People do change and their attitudes toward sin changes as well. There is no reason to know how many others they have dated in the past. Going that route only leads to jealousy.
Now if a person has had sex in the past with other people, you do want to ask the simple question of whether they have any children they are responsible for because those children will also become your responsibility if you do decide to marry.