Clusters of galaxies produce negative features at wavelengths λ > 1.25 mm in cosmic microwave background (CMB) maps by means of the thermal Sunyaev-Zel'dovich (SZ) effect, while point radio sources produce positive peaks. This fact implies that a distribution of unresolved SZ clusters could be detected using the negative asymmetry introduced in the odd moments of the brightness map (skewness and higher), or in the probability distribution function for the fluctuations, once the map has been filtered in order to remove the contribution from primordial CMB fluctuations from large scales. This property provides a consistency check on the recent detections from CBI and BIMA experiments of an excess of power at small angular scales, in order to confirm that they are produced by a distribution of unresolved SZ clusters. However, it will require at least 1.5-2 times more observing time than detection of the corresponding power signal. This approach could also be used with the data of the planned SZ experiments (e.g. ACT, AMI, AMIBA, APEX, 8-m South Pole Telescope).