Hi David,
I determined what my vocal range was because of my early experience with choral singing. I worked with the choir director on the piano and she had me sing scales with the piano to determine where my vocal range limits were. I ended up singing as a bass, though my vocal range marks me as a baritone. They enlisted me in the choir because they thought I was a tenor, but I do not have the very high range of a true tenor. My son, for example, is a true tenor. Check out
this recent video for a comparison between my voice and my son's voice.
In my early days on the guitar, I used to use the capo to keep raising the pitch of a song to see how high I could sing the song. I still do that today, looking to see where I really sound best when working out a song. None of us sound good if we are pushing our voice too high or too low.
I hope some of this helps you.
- Glenn