A method for checking the authenticity of a security article (158) wherein the security article contains magnetic particles having a magnetic saturation field Hs being greater than 100 A/m. The method comprises the following steps: (a) generating an alternating field of one or more fundamental frequencies; (b) subjecting an article (158) to be detected to said field; (c) detecting a detection signal emanating from said article to be detected; (d) examining the detection signal for the presence of any particular higher harmonics of said fundamental frequencies or of any linear combination of said fundamental frequencies and of said harmonics, said particular harmonics or said linear combination being indicative of the presence of said magnetic particles. The method is applied at point of transaction locations, where the article to be detected is brought into close proximity with a sensor.