Display device ON/OFF detection methods and apparatus are disclosed. Example metering apparatus disclosed herein include first means for sensing an audio signal output from a media device. Disclosed example metering apparatus also include second means for performing audio code detection to detect audio codes in the audio signal, and third means for determining an audio gain level applied to the audio signal. Disclosed example metering apparatus further include fourth means for determining whether the media device is ON or OFF based on the audio gain level applied to the audio signal and whether a valid audio code is detected in the audio signal.