My 150cc bike haves a drum on the back, it is easy to inspect it, it has a dial, just press the brake pedal, look how far the lever goes against the dial and you can know how much life the pads have left.
As Taz already said it, back brakes need so little power that almost anything can get the job done.
I like the simplicity of a disc brake, but it requires a lot more manteinance than a drum (replace the fluid every 6 months, inspect the lines and replace them to avoid any minimal break in them that may not be visible, etc.). In my bike's drum, just replace the pads once the lever goes too far on the dial, it is as manteinance free as a brake can be.