The overflow/expansion tank/reservoir should not get warn unless something serious happens. The radiator cap regulates pressure in the cooling system, as the engine heats up pressure increases and a small amount of warm coolant will bypass the radiator cap into the expansion tank. Then, once the engine is off and cools pressure in the cooling system will drop and create a small amount of suction which will pull that coolant back into the radiator (via the radiator cap) from the expansion tank.
If there are no leaks and you get to know your car, you should be able to identify the "normal" cold engine and operating temp coolant levels in the expansion tank. No need to remove the radiator cap, just pop the bonnet and look at the tank hot or cold.