  We describe a straightforward modification of frequently invoked methods for
the determination of the statistical significance of a gamma-ray signal
observed in a counting process. A simple criterion is proposed to decide
whether a set of measurements of the numbers of photons registered in the
source and background regions is consistent with the assumption of a constant
source activity. This method is particularly suitable for immediate evaluation
of the stability of the observed gamma-ray signal. It is independent of the
exposure estimates, reducing thus the impact of systematic inaccuracies, and
properly accounts for the fluctuations in the number of detected photons. The
usefulness of the method is demonstrated on several examples. We discuss
intensity changes for gamma-ray emitters detected at very high energies by the
current gamma-ray telescopes (e.g. 1ES 0229+200, 1ES 1959+650 and PG 1553+113).
Some of the measurements are quantified to be exceptional with large
statistical significances.
