How do you classify a singer’s voice?

I don’t! At least not right away. It’s wrong to prematurely classify a voice before you really get to know what it can do. Too often, existing range is the sole determining factor in placing a singer into a certain category. The most important factor to consider is the basic quality of the voice. Assuming that your speaking voice is clear and unforced, your singing voice should be based on the quality of that speaking voice.