There is a warning lamp when the fluid is low. At that point I would not drive the car without inspecting the brakes and depending on what you find, adding more fluid.
If the reservoir was full when the pads were new, then the fluid will drop low enough to trigger the lamp as the pistons move out. But if you keep topping off your fluids, or go to places that might do that for you, then that isn't going to turn on the lamps when the pads get low. Also if you have uneven wear, you might burn through a whole pad on just one side without dropping the fluid level enough.
So yes there is a warning lamp, but don't rely on it to tell you need new pads.