Hummm.....
some people have multiple octaves of "range" they can use... some have but one octave, and only one key to boot... another member of metallica joked in an interview they got sick of doing Eminor, as thats all the lead singer could SING IN at foirst, LMAO
I have READ that your MOST natural singing key is when you are recording talking softly and naturally, and the meter centers around one particular key...
as far as going out of range? eh, that should be obvious... non-musical listeners can usually point out where a singer "loses it" reaching too high or too low out of their range?
for picking out natural key, a "gut hunch" (and nothing else... as i cant sing one note..lol) tells me that recording yourself sober and "dry", and running it thru ANtares just to watch the pitches/key... might work...