So yea i've started writing songs and such and a lot of feed back on them has been "practice singing on key more" so basically i was wondering...A) How do you tell if you are singing on key? B)How can you improve singing on key? (links/exercises would be great)
Hands up if you thought he meant the amps?

You could get lessons or find an ear-training course.
well your ear should hopefully learn to sing in right pitch. Try to sing the same tone you are playing on your instrument(assuming you play). Play some scaled and sing along as you go. Then when yuou think you have it- play just the root chord and sing the scale!
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a) You've just got to listen and make sure that your notes are in tune and make harmonic sense... and being able to hear that will come when you...
b) practice singing all the damn time. when you play a melody on guitar try to sing along with it. Sing every song you listen to in your car, even the guitar solos. It will sound terrible at first, but you will soon be able to recognize relative pitch and dissonance between notes not in tune. A singing coach or singing lessons are good if you want to work on tone and projection and stuff like that, but singing in key is something you can fix yourself if you practice.
It basically means your not singing in tune unfortunatley, just practice using your musical ear, or just get proffesional lessons. 'Excersises' wont make a difference if you dont have the initial building blocks of singing.

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Sing along with scales and you can see if you're in key and move up to get your voice adjusted to shifting, might be a bit awkward on guitar, if you have a keyboard or piano it's easy. You'll also be able to find your range this way, take care to avoid straining over it.