In this scenario, how much do you want to know about the ex and what details do you want to know? Also, do you feel you have a right to know, and how would you react to simply being told by the person that they do not wish to discuss anything about their ex?
To be frank, that's ROCKY territory and considered "deep conversation" that can make or break your potential future.
Personally: I'm not all that interested in hearing about past relationships or talking about my own. When asked about it, I give them a short to the point to the answer...If they want to know why..."they weren't much of a giver".. "two different outlooks on life." ..and CHANGE THE SUBJECT..