Transverse electromagnetic (TEM) waveguides are predominantly used for emission and immunity tests. General requirements for TEM waveguides are given by the IEC 61000-4-20. Annex C of the IEC 61000-4-20 specifies immunity tests based on high-altitude electromagnetic pulses with a double exponential waveform. This waveform's shape is sufficiently defined by the rise time and the pulsewidth. The quality of its transmission within a waveguide can, thus, be expressed by the allowed variation of these parameters. However, other arbitrary signals cannot be reduced to just these characteristic parameters. In this paper, a method is described that offers the possibility to characterize the transmission quality of a TEM cell for arbitrary transient waveforms. It is based on the Pearson correlation coefficient of a so-called reference signal and the signals being measured within the test volume of a TEM cell. Both signals are measured simultaneously with identical field probes in order to be independent from the reproducibility of the signal generator. The signals are windowed and limited to include only the defining reflections and distortions. By means of this signal-dedicated validation procedure, the transmission quality of a TEM waveguide can be validated for an arbitrary transient waveform.