It is not a service requirement to change the pads unless they are worn below the limit. With the wheel off you can see the two pads sitting in the brake caliper. If they look like there is less than 3 mm of friction material on them then change them.
See my other work section on how to do this. Also note that the metal disc itself slowly wears out and will need replacing after a lot of miles. Check the disc for cracks, discolouration or very deep score marks. Get them replaced (as a pair) if you think they are knackered.