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.
thanks everyone! I think the current sensor is the way to go as I don't have video out, if connect anything to audio out it silences audio on the TV and the USB port is in use so it's not an option.