on domestic TVs. A little red light would go on, or "lying!" flash up at the bottom of the screen
This is getting on to being possible (though I imagine there are calibration difficulties) right now.
See, for example, the introduction at
http://campus.umr.edu/police/cvsa/patflood.htm
A new tool has been introduced that assists law enforcement in determining the truthfulness of people making oral statements: the Computer Voice Stress Analyzer (CVSA). The CVSA has proven itself to be a valuable tool for law enforcement agencies because of it's cost effectiveness. The CVSA may change the way law enforcement approaches the business of finding the truth from the spoken word.
The CVSA has definite benefits such as it: does not require a lengthy discussion of the questions used (is not restricted to "yes" or "no" answers, but rather can analyze any spoken word, or even a dull groan); eliminates the frustrating "inconclusive results" category that oftentimes was the finding of a polygraph examination; is not affected by the subject's age, medical condition, or history of drug use and can be used to analyze audio and video tapes for truth verification.
The CVSA works simply by measuring the tremors in the voice. When people lie, they place themselves under stress which causes a reaction in the autonomic nervous system. Under stress, the body prepares for an emergency which causes the blood to rush away from the limbs, and the muscles to tense. Since the vocal cords are principally muscle tissue, they too, tighten, causing sub-audible changes in the voice. These tiny changes of frequency modulations that occur in every human voice are called micro-tremors.
The variance in these inaudible and involuntary modulations are measured by the CVSA, and the results determine if a person is lying or telling the truth.
Now if you could get some Free software to do this, you could already have a crack at having it analyse the output of your TV or radio - an invaluable adjunct to political viewing/listening I think!