Some kind of light sensor mounted (eg. stuck to) the "on" power LED?
Also if the TV has a "video out" port (eg. composite video) a simple voltage detector might show if the TV is on because the active pin would be above ground potential.
Some kind of light sensor mounted (eg. stuck to) the "on" power LED?
Also if the TV has a "video out" port (eg. composite video) a simple voltage detector might show if the TV is on because the active pin would be above ground potential.