Learn the cutoff scales for each range. A soprano sings from the A note to the F or G note as her range. A mezzo sings from the G note to just below the C note. The contralto ranges from the E note to the second G note. Listen to these different ranges.
Listen to singers who are considered soprano, mezzo or contralto. Once you can identify what each of these three ranges sounds like, you will have an idea of the ranges' sounds rather than just a description of what they are.
Compare your voice to the singers in the categories. For accuracy you can make a recording of your voice so that you can compare your recording to the singers you have heard in the three ranges. Sing in a voice and tone that's comfortable and feels natural for you.