Fully automated or semi-automated scanning electron microscopes (SEM) are now commonly used in semiconductor production and other forms of manufacturing. The industry requires that an automated instrument must be routinely capable of 5 nm resolution (or better) at 1.0 kV accelerating voltage for the measurement of nominal 0.25 - 0.35 micrometer semiconductor critical dimensions. Testing and proving that the instrument is performing at this level on a day-by-day basis is an industry need and concern and is addressed in this paper. Furthermore, with the introduction of fully automated inspection and metrology instrumentation, not only does an appropriate, easy to obtain or manufacture measurement sample have to exist, but also an objective and automated algorithm developed for its analysis. Both of these have been the objects of a study at NIST and the fundamentals are discussed in this paper. In scanning electron microscopy, two of the most important instrument parameters are the size and shape of the primary electron beam and any image taken in a scanning electron microscope is the result of the sample and electron probe interaction. The low frequency changes in the video signal, collected from the sample, contains information about the larger features and the high frequency ones carry information of finer details. In principle, if the sample geometry is known, the geometric parameters of the primary electron beam are mathematically determinable from an acquired image. The method described in this paper is based on the frequency domain representation of a scanning electron microscope image and can also be used to check and optimize two basic parameters of the primary electron beam, the focus and the astigmatism. The application of this technique to regularly check the resolution of the SEM in quantitative form also is discussed.