Crank Sensor on 1995 Pontiac Grand Prix

I think the crank sensor needs replaced on my grand prix, how would I know for sure and do i need one or the one behind the harmonic balancer too. When I go to start the car, it won't turn over

One way to tell for sure is if you have a crank sensor code. This can be obtained with a scanner. then you will know for sure. what is your car not doing?