If its overfilled it will normally leak water out of the radiator cap when hot, If you want to check that the low water lamp is working, with the engine cold turn the ignition on then drain a little water out by undoing the little brass bolt on top of the radiator observing the light on the dash till it comes on, then top up till it goes out.
The expansion tank is made of black polyurethane and you can't see the level, word of warning never put any thing into the tank like a piece of wood to check the level, you can dislodge the float from the water level sensor
