"A University of Arizona professor has invented a sticker that can tell consumers if a fruit or vegetable is ripe. The stickers will be available to growers next year and should make their way to supermarkets within two to three years, said Mark Riley, a UA assistant professor of agricultural and biosystems engineering. A marker on Riley's RediRipe stickers detects a chemical called ethylene gas, which is released by fruit or vegetables as they ripen. As that happens, the sticker turns from white to blue."
A similar product called Ripe Sense is already on the Australian market and was called one of the 36 greatest inventions of 2004 by Time magazine. "The ripeSense sensor works by reacting to the aromas released by the fruit as it ripens. The sensor is initially red and graduates to orange and finally yellow."