In any scanning electron microscope (SEM) organic contamination of the vacuum chamber leads to undesired material deposition resulting in artifacts in imaging or compromises focused electron beam induced processes like etching (FEBIE) [S. Matsui and K. Mori, Appl. Phys. Lett 51, 1498 (1987)] or deposition (FEBID) [S. Matsui and K. Mori, J. Vac. Sci. Technol. B 4, 299 (1986); W. F. van Dorp and C. W. Hagen, J. Appl. Phys. 4, 081301 (2008)]. This effect can also be used on purpose as a method to evaluate the contamination level of a SEM. With a standardized process for controlled deposition from residual gas, a method to evaluate the contamination level of an electron microscope quantitatively and reproductively was developed. Additionally, this method not only allows monitoring the contamination level of a SEM over its lifetime. Also the impact of various deposition parameters on the extent of contamination deposition has been investigated systematically. This method also allows comparing the status of different tools. A comparison of three different SEM tools of different vendors and with different fields of application is demonstrated.