is there a cheap sensor or method on how to detect ethylene from fruit?
No. Detecting small amounts of a specific gas like ethylene requires expensive equipment.
See this recent review article: http://aob.oxfordjournals.org/content/early/2012/12/12/aob.mcs259.full
The way organics are usually done is
first it goes through a gas chromatograph then to a mass spectrometer.
You could by a nice Tesla for the price of this setup, not counting
maintenance to keep it tuned and running well.