Home Automation using Arduino and Android
Typical, commercial domotics / home-automation systems do not use the standard mechanical on/off switches, but smart switches. The smart switches, usually have a dedicated, tiny microcontroller, that is able to interact with the main domotics controller, to request the associated appliance to be turned on / off.

Standard 1-way mechanical switches need to be always left in the ON position for domotics contol to work. Turning such switches off, overrides the domotics controller's view of the last operational state. So, the only way is to have a feedback mechanism using some kind of a sensor (usually a current sensor, but could be other kinds - e.g. a simple LDR for light sensing), that reads appliance state, and notifies the domotics controller. 

PS> Standard warnings regarding the hazards of working with AC line voltages apply. However for the purpose of this answer, lets assume that you know what you need to know.