In his book Reaching the Nations Mike Frisby helps churches work with people who are engaged in cross-cultural mission. In the chapter ‘profiling the personnel’ Mike says:
While the old saying still holds true that “availability is more important than ability” and Scripture teaches that “the Lord chooses the weak to confound the strong”, nevertheless, there are some basic qualities one is looking for in those offering themselves for cross-cultural work. (See Judges 6:11-16, 1 Samuel 16:1-13, 1 Corinthians 1:26-31)
Good training can develop character and skills, and good teaching can rectify a lack of knowledge. However, serious flaws in any area will not disappear simply because the person is transported into a new and exciting environment! As a general rule of thumb, if you would not be happy giving responsibility to them and using their gifts in your own local church, then they are not ready for cross-cultural work at home or in other nations.
Mike then gives a whole series of questions to help you assess a worker in the areas of character (creditable, dependable, teachable, generous, natural, fruitful & resourceful); relational skills, family, health, work experience & life skills.
The questions are thoughtful, careful and very, very helpful. You can get the book here