When dating asking about past serious relationships can save you a lot of time about continuing on.
When dating and you are interested in a serious relationship, it is okay to ask about past serious relationships. After the initial attraction, then there is the rest of the story. Past relationships can tell you a lot about whether you want to continue forward with someone. You don't have to be an investigative reporter when you ask. It can happen very naturally asking about how long the person has been single. Then ask how come it didn't work out. How the person answers this question can be very telling. Most people usually describe what wasn't working with their ex-partner. (What was wrong with them). Which is okay. However what you also want to hear is, their description of how they were in the relationship. In other words how they contributed to the relationship not working and their insights about themselves and what they learned about themselves and the changes they have subsequently made. If they just comment on their ex-partners short-comings and nothing about their part in the relationship not working. Then don't continue with this person; because they are doomed to repeat themself. Ask about all serious relationships and how come they didn't work out. If you hear it was the other person's fault and no ownership from them. Then say it was nice meeting you and move on. By the way, you should be prepared to answer the question about past relationships with insight about your part in it not working out. This is called being transparent, honest and open to growth and change.