If you can’t tell from the price tag, your best bet may be isotopic composition analysis, of course:
Just so everyone knows how ridiculous this is: The way they can tell organic food from non-organic food with relatively descent condifence is by putting it through a series of chemical reactions that reduce it to purified hydrocarbons. They then break down these hydrocarbons to ultra-pure carbon which is then seperated into individual atoms which are ionized and accelerated through a series of magnetic fields. The heavier isotopes tend to be less deflected than the lighter ones. So they shoot the carbon ions through the magnetic fields thousands or millions of times and count how many are deflected by a given amount. Then they compare the ratios and determine if you were ripped off on your designer $12 a gallon milk.