If she has taqwa she will be honest....
If it matters to you that much, then you must ensure you find out the truth.
If it cameout after marriage are you strong enough to forgive her? if not then you need to find out now.
I think when you meet her, just talk about the importance of Honesty to you, explain your views on witholding information that may effect the relationship. give past relationships as an example... and say to her, for me its really important and list why. And tell her if i was to find out after marriage that my partner had a relationship before i would do this.... and tell her honestly what you would do. Be honest you wont accept anyone like that. At least you will put the fear in her to be honest otherwise its curtains for the marriage.
Instead of being so direct, have you had a relationship. put it a different way,.... have you always been so practicing? when did you start to properly practice... let her answer... maybe she will say something along the lines... no i was not always practicing and you can question further, how so? in what way? see what she says... heranswer should lead you onto another question... you can find out quiet a bit that way.
And if your still unsure go to her freinds... visit place she used to work go to school at. Get references from teachers, managers.... they would describe her character, she was a shy, modest student, or she was loud and outgoing and socialized alot. You will find out soooo much. If it is that important to you, then you gota do your reasARCH. also do not tell her before, as hadith stated you can "spy" or observe people without thier knowladge inorder to see them in thier natural state.
All the best, inshallah i hope all goes well. I am married and had to go through all this, very tough and difficult but worth it in the end. You start your marriage clean, no worries about the past and get on with life. Alhamdulillah.