I often use the symmetry of the time series for the Z coords of the P atoms for each leaflet as a quick gauge of drift through the course of a simulation. It's a quick and easy analysis with CORREL, indicated by the commands below--

:
define leafa sele type P .and. prop Z .gt. 0. end
define leafb sele type P .and. prop Z .lt. 0. end
correl ...
enter lfa atom Z sele leafa end
enter lfb atom Z sele leafb end
: