The invention is intended for illicit drug detection, preferably to Cocaine detection in street samples containing in addition to Cocaine also adulterants and/or diluents (also denominated as cutting agents) used for increasing the quantity of the product and/or for disguising the existence of Cocaine. The invention provides preparing of the liquid street sample, taking an aliquot of said sample, its analysis with help of SFS (Spectral Fluorescence Signatures) technology, fixing the result of analysis as a reference value, the subsequent acidification of the liquid sample, taking an aliquot of the acidified liquid sample and its analysis with help of SFS technology, fixing the result of analysis and comparing said result with the reference value. The result of comparison enables to differentiate between Cocaine Base and Cocaine hydrochloride in the street sample.