Henry_Best:
I get the impression from his post that he wants to use a two way switching system with one of the switches controlled by an Arduino. He needs a double throw relay and a double throw (two way) switch for that. There is then no need to detect what state the relay is in. Throwing the two way switch will invert the state of the relay.
Ah, but it is not the state of the "two-way" relay that he wants to know, but whether the appliance is in fact on or off.
Thus my point - the controller and relay exclusively controls the appliance, then there is no doubt as to its state, and you direct all forms of switching through that controller.
If you are going to use a controller - as for my house lighting - best do the job properly, "from the ground up". I challenged the concept that you would need some sort of "alternative" control to the controller itself.