First of all, you have to recognize the tonal center of the song. Depending on the difficulty or complexity of the accompaniment, you might need to listen to the choir and find out what chords they are using. Normally you can listen for certain 5-1 or 4-5-1 ( or interval 7-1) progressions to hear the tonic (the tonal center). From that point, you can determine if it is major or minor by the surrounding intervals or sounding pitches. If your ear is really strong, listen for the tonic chord. If not, listen to the choir and develop a key off of the notes they are singing. Hope this helps and God bless.
