Are you 6 foot tall or over?
IMO leg presses are shit, all they do is irritate my lower back.
You can tell if they are benefitting you or not. Don't just listen to your ego and think 'wow I'm using 500LB's', gains from the leg press don't really translate over to the squat, at least not for me. If you feel new strength developing and see new muscle then they are obviously benefitting you.
Whilst you shouldn't give up on an exercise immediately, don't be afraid to jettison the leg press if it is not producing...
Good luck wit your training.