Remember those test patterns you see in the middle of the night on television. Televisions stations called it the night. Either it is the color bar pattern or with black and white, the Indian head. Also, you hear a tone played at a certain frequency, music, or feed from a radio station.
Indian Head
SMPTE Color Test Pattern
[youtube=425,350]http://www.youtube.com/watch?v=kufKgNqJR9U[/youtube]