Think of it this way:
- If she gets the visa and comes to USA, is there anything besides your/her desire, which would prevent her marrying you as soon as she comes to USA?
- If she gets the visa and comes to USA, and decides to stay, would there be any issues in her country?
If the answers are "nothing" and "no", she is unlikely to get the visa unless her circumstances change.
If the answers are different, you have your piece of evidence. Write it down, and provide with your visa application (you can bring it to the Consulate printed).
She is applying for a nonimmigrant visitor visa, and she need to provide supporting evidence that she is coming for a visit, and will return back to her country of origin. However in your case she has another complication because you mentioned she's your future wife. The consular officer would assume that she intends to marry you in USA and won't come back. Thus the visa application should be supported by the evidence which would counter this assumption. For example, if one of your divorce hasn't yet been finalized, or if you have already scheduled - and paid for - the wedding in a different country. The evidence has to be something the consular officer can see and check themselves, not promises.
Finally, there is a way to really be sure that your application is watertight, and has the best chance of success. You can retain an immigration attorney who would help you with this. This is still not a guarantee she'd get the visa, but a competent attorney would help you preparing the best application - and would advise against applying at all if he sees your chances are weak. Just make sure you are dealing with an attorney licensed in your state, and not an "immigration consultant" which could be anyone.