not sure how to explain. are there any lessons on here about knowing what key a singer is singing in and what chords to use for it?
This is pretty much the only way. Or, you could try holding a tuner up to it. Just don't do it to the X-Factor because noone sings in key on that.
try this, sing and hold a note, a random note,
then on your guitar try and replicate that note being hummed

get good at doing this via practice, and then create small melody lines, and replicate the notes on the guitar

down the track with MUCH practice, you can do both at the same time as you can pick out the relative pitch of the next note
Quote by Ikonoklast
Or, you could try holding a tuner up to it.

I actually have been known to do this to illustrate to myself what a limited, incomplete, and uncontrolled vocal range that I have. No wonder I can't hold a tune - I can't even hit the right notes.