At least a component of a package (10) includes at least a first plastic (e.g., high density polyethylene HDPE, polypropylene PP) having a first elecromagnetic energy absorption spectrum and, dispersed in the first plastic material, a second plastic (e.g., ethylene vinyl acetate EVA) acting as a visually non-detectable marker and having a second electromagnetic energy absorption spectrum. In order to authenticate the package, electromagnetic energy, preferably infrared energy, comprising at least one wavelength absorbed by the second plastic is directed onto the package to detect presence of the second plastic material.