Pete. There are ways of monitoring the state of A/V equipment. As Daniel mentioned, these types of products can be expensive and are normally a peripheral of a home automation controller and part of a home automation system. Probes are connected to the product than sense the current status of A/V equipment. But, there are less expensive ways to do this. One is an AC current sensor. It detects current flow in the TV's power cord. When the TV is drawing current the sensor will activate an X-10 Powerflash module that will send an X-10 ON signal. When the current draw stops, an X-10 OFF signal is sent. So the state of the TV can be determined by checking the status of the X-10 unit code as being on or off.
RCI Automation[Link: ourworld.compuserve.com]