Do everything as usual, but put aptitude in simulation mode, causing it to
never do any changes (except update and autoclean, which should both be
harmless) to your system. This is especially useful on a new system to make
sure pkgsync behaves as expected.
Note that aptitude prints out its intended actions _before_ running the
conflict resolver. If there's a conflict somewhere, chances are that the
results on your system will be different from what aptitude prints out.