Warm up by buzzing the first five notes of a major scale. Generally, you can start on middle C, if your voice is low, try starting an octave lower.
Determine your vocal range by starting with middle C on the piano and singing as high as you are able to comfortably. Record the pitch and then sing downwards from middle C as low as you can sing comfortably. Do not strain yourself or push to get a higher pitch.
Sing a C major scale and notice the point at which you feel a change in the way you sing. For instance, when you sing in the lower part of the range, you will feel your voice in your chest. As you move upwards you will notice that the voice tends to move to your throat and then head. This transition is critical, make a note of the pitch that your voice changes in tone quality and physical sensation.
Determine where your passaggio falls. If the point at which your voice changes from chest to head voice falls between D and E-flat on the bottom of the treble clef, you are likely a soprano. If it falls lower than that, then you are an alto.
Use the high pitch from Step 1 and if you can sing above an A comfortably and your passaggio matches that of a soprano, you are a soprano. Also, a soprano has a lighter, more narrowly defined sound than an alto. If your lowest pitch goes down to an A or lower between the staff, you are likely an alto. The same guidelines apply to men's voices, but they are an octave lower and classified as tenor and bass.