You might want to start off with songs that have lots of unison like "I'm Available to You", "Shabach", etc.
As far as finding out whether someone is an alto or soprano, you can have each one match pitches with you as you play the piano. In the choirs I work with, I expect sopranos to be able to hit a Db or D at the least. Altos should be able to hit at list a Bb. I try not to make the tenors go higher than a G.