Brake pad thickness is easy to check visually, although it sometimes requires a small flashlight which should be part of your tool kit anyway. It is pretty obvious when a brake pad is getting thin, but even so most have wear indicators. Shown below are pads off the same bike. One new showing the wear indicators and the other worn almost all the way down to the metal.
Here is a typical brake caliper mounted on a bike for reference.
There are minor variances on how to check the pads on different bikes with different calipers, but the basic procedure is the same.
You can get a good general idea of the condition by just looking down the end of the caliper as illustrated.
This is obviously a new set of pads...if they were worn they would be riding much farther from the rotor.
If you want to take a look at the actual wear marks you can look down the top. They are kind of hard to see on these Brembo calipers, but most imports have the pin on the side so it is much easier.
