Online sites like TV Guide and Hollywood Reporter offer information regarding the salaries of television celebrities. The sites are updated annually, but not all celebrities are listed.
Generally, these and other sites run a feature article on celebrities starring in the top shows on the major networks. For example, when the top three cast members of "The Big Bang Theory" received substantial raises in 2014, increasing their income to $1 million per episode, it was big news on the entertainment websites. The sites then examine how these salaries compare to other television stars's paychecks. They obtain the information from the stars themselves, from agents or other sources.