how can you tell it's time to change the clutch

sometimes after some hard driving I can smell a burnt rubber kind of smell coming from the car.. I assume it's my clutch. I gotta learn to keep off the throttle before I downshift.. I know I should do heel toe downshifting... I'm a newbie and I'm learning..

anyways.. how can you tell when it's time to get a new clutch?
and typically how much is it to get a heavy duty one with installation included?