Methods and apparatus to detect operating states of a display based on visible light are disclosed. An example device to detect an operating state of a display includes at least one optical sensor and a logic circuit. The at least one optical sensor is disposed to detect visible light emanating from a screen of the display and to convert the visible light into an electrical signal. The logic circuit is coupled to the at least one optical sensor to generate an output signal indicative of the operating state of the display based on the electrical signal.