The Chen-Kao reaction (named after Ko Kuei Chen and Chung-Hsi Kao, Madison, Wisconsin 1926) is a chemical method for determining the presence of pseudoephedrine, ephedrine, and similar phenylalkylamines.
The reaction is used in spot tests and is also known as Chen-Kao test (or simply as Chen, Test T in UN Precursor Test Kit). 
The test is often used to distinguish ephedrine, pseudoephedrine, norephedrine, cathinone and methcathinone from amphetamine and methamphetamine,
which do not react with Chen’s test reagent.