Mordecai,
That's a real tough one, and I give you a lot of credit for dealing with it head-on.
As some have suggested, there are few guarantees in this area. I know one couple where the husband was an upcoming lay preacher in their denom, but he suddenly lost it, began beating his wife and committing adultery. They divorced. Others have married non-Christians and won them to the Lord eventually. But the first example is an aberration (I hope), and the second is very risky.
In 1Cor 7, Paul essentially tells those in mixed marriages that if your marriage is good, you as the Christian are under no obligation to end it. He says not to worry, the marriage is still holy, as are its fruit, the children.
But that's for people who are already married. To go into a mixed marriage knowingly? That is risky.
Another danger - that your fiancé will seem to convert to Christ but at root will be doing so for the wrong reason - to prevent the marriage from not taking place. Chances are good that such a conversion would unravel sooner or later. No matter what the outcome, keep communication with you fiancé' honest.
I think that if I were in your shoes, I would not go ahead with the marriage until I was sure God's blessing was on it. Ideally, that would mean that your fiancé genuinely received Christ as Lord. As powerful as human love can be, it is going to be a burden to share your life intimately with someone who does not have the same Spirit as you. There will always be a part of you - the most important part - that you will not be able to share. And that could lead to significant disagreements as to how to raise kids, career choices, etc.
Ultimately, these is a question that only you can answer.
In John 7.17, Jesus gives us an important insight:
If anyone's will is to do God's will, he will know whether the teaching is from God or whether I am speaking on my own authority.
This is an important principle. If you want to find truth, and by extension, God's will, the first step is to ensure that you would be open to it should you hear it. If you approach God with a heart sincere in its desire to please Him, that exalts Him above all else, including even the most dear human relationships, He is going to reveal His will to you, and also give comfort and strength to perform it.
Here is my prayer for you:
You learned the gospel from Epaphras, our dear fellow slave a faithful minister of Christ on our behalf who also told us of your love in the Spirit.
For this reason we also, from the day we heard about you, have not ceased praying for you and asking God to fill you with the knowledge of his will in all spiritual wisdom and understanding, so that you may live worthily of the Lord and please him in all respects bearing fruit in every good deed, growing in the knowledge of God, being strengthened with all power according to his glorious might for the display of all patience and steadfastness, joyfully
giving thanks to the Father who has qualified you to share in the saints inheritance in the light. -Col 1:7-12
I pray that as you are filled with love in the Spirit, the Lord will fill you with all spiritual wisdom, and show you the way to go that will both please Him, help others, and bring much spiritual fruitfulness. May you have His peace that passes understanding as you approach Him for wisdom in how to deal with this situation.
Rejoice in the Lord always; again I will say, Rejoice.
Let your reasonableness be known to everyone. The Lord is at hand;
do not be anxious about anything, but in everything by prayer and supplication with thanksgiving let your requests be made known to God.
And the peace of God, which surpasses all understanding, will guard your hearts and your minds in Christ Jesus.
Finally, brothers, whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things.
What you have learned and received and heard and seen in me-practice these things, and the God of peace will be with you. -Phil 4:4-9
Lastly,
here's a link to a doc I uploaded about George Muller's method for finding God's will. It may be helpful.