how do i know if they need changing?

 

Generally, with any discs front or rear, you go by the condition.
If they are warped then replace, not worth getting them skimmed.
If they have a large lip around the edge, and the pads are low, then prob worth doing the discs at the same time.
Officially there will be a minimum thickness of the discs (I dont know what this will be, but you can compare it to the thickness of new discs), and below that figure they should be replaced.
Also, if the car has done a lot of miles and again the pads are due then might be an idea to replace discs.
I always go by eye, if they look bad and have a lip round the edge I change them.
Hope that helps
Bod