It can be rather loaded question. Of course you will find out sooner or later. It just sounds like a bad start to bring up why she was divorced when you start seeing someone. And if you find out she was divorced for the wrong reasons it's like "bye-bye, I'm no longer interested". Are you to tell her: "Sorry, your divorce was not biblically right. I love you but can't see you no more." Maybe they just didn't get along, but that is no reason enough. She might have been very young when they married, and looked for the wrong things in a partner. I don't know.
While this might seem to be a touchy topic and the kind we would normally not endeavor to have with others; dating and, potentially marriage, is not a "normal" situation. It's not the same as sitting around the water cooler at work chatting, for instance.
Everyone, most especially those who have already had a failed marriage, know just how important it is to make the right choice in a spouse.
Also, it's not about being judgmental. It's about deciding on what kind of a person you want to bear and raise any future children you may have, as well as the person you're entire life will one day revolve around - an important and daunting task of choosing in which God has given us a few guidelines to follow which helps us make a better choice.
As far as the potential of having to say "I don't think you're divorce is biblically justifiable, in my current understanding of Scripture" You can certainly word it however you like, though I might rather say "the circumstances of the divorce aren't something I'm comfortable with as a Christian who is considering marriage to you" however, it's likely either way your beliefs will come to the table.
In which case, you have to determine how important your faith is to you. Standing before a broken world it we aren't going to be everyone's friend, especially when we declare our beliefs... And do you really want to marry someone who you feel you can't stand up for your beliefs in front of?
It's Christianity in action my friend. Sometimes it's time to stand on your faith in front of very real people face to face. Be kind and full of grace, but be true to God.
As for timing, sooner is always better than later. Otherwise you're just potentially wasting both your time and her's.
P.S. - I TRULY BELIEVE - what occurred before she was saved I believe should be forgiven as God forgave us.