Here are some things that I do to help individually assess students' voices:
1. I have two students at a time stand by me at the piano with their backs to the class. The whole class sings, but I can clearly hear the two students singing. Sometimes I have the class sing the chorus and the two students sing the phrases. I just got a brand-new video camera, and I have been using it to videotape the two students singing. Videotaping is nice because I can go back and listen to it when I have more time. (Audio taping would also work in this way.)
2. The students get into groups (usually four or five students), then I assign each group a number. We all start out singing together, but then I will flash a card with a number on it. The group with that number is the only group that sings. If a number is not raised, everyone sings.
3. I have a small stuffed dog named Mr. Pooch who helps with assessments. The class sits in a circle and sings a song. As the song is being sung, Mr. Pooch is passed around the circle. When the music stops, whoever has Mr. Pooch sings the next phrase. Then I push play, and the entire class sings the phrase and continues to sing until the music pauses again.