Find a place to audition prospective members of the church choir that is most like the normal singing environment. While some singers perform well in concert hall acoustics, their voice may sound significantly different in a small chapel setting.
Test the church choir applicants in a number of different songs in order to see how well they adjust to different singing demands. Have them sing scales as well.
Ask the applicants what their philosophy of singing is and how it applies to their faith. This is a way to get to singers' inner motivation for wanting to be part of a church choir.
Have the applicants sing with the general choir body to see how their individual sounds integrate in the larger singing body. Sometimes certain idiosyncrasies undetectable in solos becomes clear when the choir sings as one.
Remind the church choir applicants that attendance at rehearsals and commitment to showing up for every singing engagement is required. A choir without a dependable composition loses its ability to deliver music in accord.