Low molecular weight oligomers of epoxy resins of bisphenol A type are common sensitizers. For demonstrating the presence of sensitizing oligomers of these resins, two simple methods are described. The first one, a colour reaction, demonstrates the presence of the bisphenol A skeleton. If this test is positive, thin-layer chromatography is carried out to demonstrate the presence of low molecular weight oligomers of epoxy resins. Some practical applications are reported.