There are company's who do just this. Find out what people are watching. Nelson is one. There are two basic ways. One is to have volunteer families attach a device to their tv's This monitors and reports the programs watched. The other way is to randomly call homes at predetermined times and ask what they are watching right then. All the information is compiled and a winner for a particular time is determined.
edit.. if more than one television is being watched, all the information is submitted