What's the chemical difference between normal coffee beans and Kopi Luwak?
Actually they recently came up with a reliable test for civet coffee. Apparently a great deal of supposedly authentic stuff never actually traveled through the bowels of a cat.

According to the abstract, the levels of citric acid, malic acid, and the inositol/pyroglutamic acid are so much higher in kopi luwak that their ratios can be used to validate mixes with other beans where the proportion of kopi luwak is 50% or greater.