Have a look at them, first. Usually you can just eyeball them, though you may get a better view if you (or someone else) can squeeze the brake lever while examining them. If you do have to remove the pads to get a good look, it's an easy task. It's not rocket science: you'll know if there's enough pad remaining; if there is enough, then it's one (or more) of the things dwmckee mentioned.

FWIW, I replaced the front pads on my main gravel bike (which has seen a lot of long, steep, and dirty descents) at about 7k miles. On my other disc brake bike, I have not replaced any pads yet in almost 12k miles.