  Methods to classify events experimentally according to collision geometry are
well established and non-controversial when collisions of large ions are
studied. However, high luminosity data from $p/d$+A collisions at RHIC and LHC
provided some surprising results that either call for new physics or question
the applicability of the established methods of event classification for them.
So far there is no consensus in the community what is the proper model and
procedure to determine centrality, how to connect observed event activity with
collision geometry in very asymmetric, $p/d$+A collisions in the same sense and
with the same accuracy as was done in A+A. We argue that high $p_T$ direct
photons offer an {\it a posteriori} test of any method suggested to categorize
$p/d$+A -- and in general, very asymmetric -- collisions: the method is only
viable if the nuclear modification factor for high $p_T$ direct photons is
about unity for all centrality classes.
