The efficient and productive operation of a Concentrating Solar Thermal (CST) plant depends upon knowledge of the mean reflectance of the solar collector field, as the levelized cost of electricity has a one-to-one ratio with reflector optical performance. Considering the size of such collector fields, a continuous detailed survey of the whole field is not a practical proposition. This paper presents a statistical approach to measuring mean field reflectance by sampling only a subset of the field reflectors. We find that three samples are sufficient for evaluating the mean reflectance of a single facet, and that the locations sampled within the facet do not impact the estimate of the mean. Further, for any collection of facets behaving as a cluster, again sampling three facets is sufficient to yield an estimate of the mean reflectance of the cluster. These sample numbers hold for an error of 2.5% at 95% confidence; different error or confidence intervals will affect the sample numbers required. The results are obtained from studying the behavior of both a heliostat and a linear Fresnel collector field. A blueprint for large CST collector fields is derived.