Read the vocal score to determine the parts needed. Many vocal sheets include only a melody line, typically with piano and guitar accompaniment for the harmony, but in more complex vocal music, the vocal arrangement will include both the melody and harmony. A typical vocal arrangement will have soprano, alto, tenor and bass parts, indicated by four attached music staves and S, A, T and B at the beginning of each of the staves to indicate the soprano, alto, tenor and bass vocal parts.
Read the ranges of each vocal part to determine vocal parts. Soprano range is from middle C on the piano to the C written two ledger lines above the treble clef, a range of two octaves. Alto ranges from F below middle C to D on the fourth line of the treble clef. Tenor range begins on the second space of the bass clef and spans to octaves, to the A notated on the space above the third ledger line over the bass clef. Bass ranges from the E notated one ledger line below bass clef to the E two ledger lines above the bass clef. Spotting these ranges in sight reading can help you determine vocal parts even when there is no letter indication of the part.
Read four vocal parts together to determine a song's harmony. The combination of the separate parts will typically form chords in choral arrangements. Alto or soprano will typically carry the melody, with the other handling a harmony note. Tenor and bass will almost always harmonize with chord tones. Sometimes the parts have individual music staffs; sometimes soprano and alto share a staff, as do tenor and bass, with the parts written individually.
Read alternative vocal part markings to determine special circumstances or directions for a vocal arrangement. The phrase "mezzo-soprano" tell you to sing in between soprano and alto. "Contralto" typically refers to a female voice lower than the normal alto range. An indication of "coloratura soprano" doesn't change the range of the soprano, but indicates using a voice that is even softer than a typical soprano.