I have a decently low singing voice and I was trying to sing along with some keys to see how low I could really go and when I did it I was disappointed with the results but then I did the exact same thing with a friend and when I stopped, saying I couldn't go any lower, he said I already had been. Do I hear my own voice differently or something? How can I really tell where my vocal range is?
1) Yes, one hears the own voice at a different pitch, which is one of the reasons, why it is quite hard to recognize it from recordings. Problem is, that sound you hear is not only transported via air but also via the bone of the skull. Putting the flat hand on the side of the face, so that a "V" formed by the fingers with some fingers on each side of the ear is used by some singers during rehearsal to get a more neutral impression.
2) Using an electronic tuner or app should work if you can produce the tone somewhat constant and long enough for the measurement.
Before singing. Learn to confidently recognize pitch.
Example: Some one plays notes ( one at a time ) on a piano. As each note is played ; You identify if the note is higher or lower. No peeking ! (ha) Do this until you become strong and confidant at this.