Is there a detection method available to detect a plane without using RADAR? If we don't have RADAR, is there any other method to detect flying objects?

All flying objects disturb the air around them. Is there any method by which we can detect that this air turbulence is from an artificial flying object or a bird etc.?
Radar is, by far the most widely known and used of the active detection methods.

Passive methods include the use of visual and infrared search and track systems such as the F-14's Television Camera System or the Eurofighter Typhoon's PIRATE infra red search and track systems.