brakes

Whilst the pads are still new have a good look, sighting down the gap between the disc and the calipers. It would probably help to take the pads out, have a good look at them and put them back. Once you are familiar with what you are looking at you will be able to see the thickness of the pads with them still in place. Oh, and take the opportunity to grease the retaining pin thread (for Shimano).

I did none of that! I found out my pads were worn to the bone when braking started getting a bit iffy and the disc was a little scored. Things also sounded different. Whilst replacing the pads (easy) I figured out the above.

It's quite easy. In decent light, perhaps your phone to help, with the wheel in place, squeeze the brake and look down to the pad. You can normally see how much pad is left, looking for the lip. You want more than a mm both sides.

Pads don't always wear evenly, pistons can get lazy.

The main clue you've left it too late is squealing brakes, not just a squeal when first braking when wet where they then clean up after a couple of presses, but permanent squealing. Change them quick because they don't work as well and score the disc.

It's easy enough to pop out the pads and check these days, just make sure you have something to prise the pistons apart if you need to replace them (there are special tools but a big screwdriver used carefully works - when I know I am replacing pads, use it on the old pads before taking them out and you don't need to worry about any damage as it needs quite a bit of effort to open them). Also, I've noticed nervous riders putting a wedge in the brakes when changing a wheel roadside to avoid accidentally closing up the pistons. As long as you are putting the same pads back, you don't need to worry about opening up the pistons unless you've touched the brake.

The brakes have a self-adjusting system where when open, fluid can return to the reservoir, so you can press the pads back, but when they are pressed, the circuit seals and they have a cunning system where they allow the lever to add more fluid into the circuit if needed, but that fluid doesn't get out again by itself, but can be returned by pressing the pistons open when the brake is not pressed.

Steve K wrote:

Having recently switched from rim to disc (and being delighted by the switch) my big question is how easy is it to tell when it's time to change pads.

Your braking deteriorates gradually so just consciously check the braking every once in a while.   Well over a year based on my experience (5000 miles pa).