How to tell when disc brakes are worn?
You need to replace your pads when they are about 3mm thick including the backing plate. You should be able to see how thick they are with your wheels off but you might need to remove them from the callipers to be sure. See the manufacturers site for details of how to remove and fit the pad - its normally an easy job.s

If you do need to change the pads on hydraulic brakes you need to make sure the pistons are all the way back, otherwise there won't be enough of a gap between the pads and the disc. Its best to do this while the old pads are fitted by using a flat bladed screwdriver to push them apart.

The other thing to remember is that the new pads will need bedding in before you use them in anger.

If you do wear the pads all the way down they won't stop you very well and they will make a mess of your discs.