We introduce a new and straight-forward methodology to accurately determine the Pt content in polymer membrane electrolyte fuel cell (PEMFC) catalysts consisting of carbon supported Pt nanoparticles (Pt/C). The method is based on an indirect Pt proof (IPP) consisting of the oxidative removal of the carbon support, the digestion of the Pt in aqua regia followed by a replacement reaction to form Cu ions (CuCl₂). The Pt content is then determined via the Cu-ions with the help a complexometric indicator using a simple titration. The procedure is fast and does not require any expensive equipment. Thus, it can be implemented in any standard chemistry laboratory. The advantages and disadvantages of the IPP method are evaluated in a comparison to alternative methods for the determination of the Pt content in supported catalysts, i.e. inductively coupled plasma mass spectrometry (ICP-MS) and UV/Vis spectroscopy (UV/Vis). It is demonstrated that the IPP method delivers reliable and accurate results and is less influenced than for example ICP-MS by side effects such as excess in nitric acid or organic impurities. Furthermore, during the procedure up to 60% of the Pt material is recovered during the IPP procedure.