Ask her the questions below.
1. Make sure the sister has the correct understanding of Tawheed. (see the Fundamentals of Tawheed by Bilalal Philips)
2. Make sure the sister believes and understands in the five pillars and the six articles of faith?.
3. When the sister comes to meet with you check to see if she is correctly Islamicly dressed.  If she is not, then be mindful that she may not know the requirements and  it may take time for her to adjust to the proper understanding.
4.If  she is a follower of any groups that have deviant practices or if she is following a group whose practices put them OUT of Islam you may rethink marrying her. Some of these groups are Nation of Islam, Shia, Ahmadiayah, Rashad Khalifa followers, Qadianis, 5% Nation, 12ers   and other unnamed groups that are well known among the believers to have problems with their aqeedah
5. Ask her if she understands blind following of the Madhhabs
(See Blind Following of Madhhabs by Shaykh Muhammad Sultan al-Ma'soomee al Khajnadee)
6. Some "scholars" have a modernistic slant, ask her which scholars she regularly refers to for Islamic materials and /or answers. 
7. Has she pledged bayah to any one? If he has, this is a misunderstood practice, and you many need to talk to him about it. See Huda Magazine.
8. Ask her, do you try to lower your gaze when looking at other men? Do you needlessly chat with men?
9. Will she expect her  husband  to be in the company (mixing and mingling casually) with  other women in your home or while visiting others, including her family ?
10. Do you pray all the prayers on time, all 5, and the Sunnah prayers?
11. Do you fast all of  Ramadan (i.e except when you are prohibited)?
12. Do you engage in voluntary fasting?
13. Do you read Quran every day?
14. Do you remember to say your duas at certain times, like before eating, after eating, at time of sleep, when traveling, etc.?
15. Do you refrain from backbiting and slander?
16. Do you refrain from idle talk?
17. Are you careful to check your grocery products for haraam ingredients?
18. Are you able to divide your time reasonably between (house)work and Islamic activities and your family?
19. Have you performed  Hajj and/or Umrah?
20.Many scholars consider Music and photography (still and moving pictures, 1,2 and 3 D included)  Islamicly unacceptable. Ask her if she partakes in these activities. Bukari# 7.110 Narrated  by 'Aisha: " I bought a cushion having on it pictures (of animals). When Allah's Apostle saw it, he stood at the door and did not enter. I noticed the sign of disapproval on his face and  said, "O Allah's Apostle! I repent to Allah and His Apostle. What sin have I committed?' Allah's Apostle said. "What is this cushion?" I said, "I have bought it for you so that you may sit on it and recline on it." Allah's Apostle said, "The makers of these pictures will be punished on the Day of Resurrection, and it will be said to them, 'Give life to what you have created (i.e., these pictures).' " The Prophet added, "The Angels of (Mercy) do not enter a house in which there are pictures (of animate objects)."Hadeeth 7.494B Narrated Abu 'Amir or Abu