I basically know what my voice's top note is without straying badly... I just adjust the key of the piece so THAT is the top note of most songs.
I also do that Diki, but along with the highest note, I have to look at the 'vocal path' I have to take to get there ... if there is a phrase with high notes close to the top of my range just before I have to hit the highest one, that could be a little more difficult to control the breathing properly ...