Does the TV have any output jacks, like video out? If so, then you should be able to rectify the sync pulses coming out of the jack and produce a DC voltage. If the TV is off or stolen, the jack will not have any signal and the DC voltage will drop to zero.
Another option is to sense the AC current going into the power cord. Basically, you loop one wire of the AC line through a toriod coil. The coil's output is dependent on the current through the wire. Again rectify the AC to a DC voltage and you have an on/off indicator. You can use an extension cord to split the wires apart, so as not to void your warranty. (If you wrap both wire through the toriod, the the net AC field is zero.)