Old fashioned way: Tape a CDS cell to a NE lamp, connect one lead of the lamp to the return side of the power cord, connect a 10M ohm resistor to the other lead of the lamp, connect the other resistor lead to the appliance feed side of the main switch (your breaker), connect one side of the CDS cell to +5 V on the arduino, connect the other CDS cell lead to an analog in on the arduino in a voltage divider config. determine the analog level when the switch is on. in your software, then the analog in is at, or above, the above found analog value (less some very small amount), the switch is on, the CDS cell conducting and resistance lowered, thus imbalancing the voltage divider's condition when the switch is off. You cann't get much more caveman than that.....