  We present a diagnostic test of possible Galactic contamination of cosmic
microwave background sky maps designed to provide an independent check on the
methods used to compile these maps. The method involves a non-parametric
measurement of cross-correlation between the Faraday rotation measure (RM) of
extragalactic sources and the measured microwave signal at the same angular
position. We argue that statistical properties of the observed distribution of
rotation measures are consistent with a Galactic origin, an argument reinforced
by a direct measurement of cross-correlation between dust, free-free and
synchrotron foreground maps and RM values with the strongest correlation being
for dust and free-free. We do not find any statistically compelling evidence
for correlations between the RM values and the COBE DMR maps at any frequency,
so there is no evidence of residual contamination in these CMB maps. On the
other hand, there is a statistically significant correlation of RM with the
preliminary WMAP individual frequency maps which remains significant in the
Tegmark et al. Wiener-filtered map but not in the Internal Linear Combination
map produced by the WMAP team.
