if you instinctively go to snowplow to stop or slow down you're not ready yet.
if you ski parallel with your feet farther apart than your shoulders you're probably not ready yet.
if you're skiing in the back seat (ask a friend), make sure you consciously avoid it cause you're not going anywhere with backseat habits.
but once you get into the park it's almost the same as snowboarding. only difference really is being forward/backward instead of sideways.
you'll also probably find it easier to land on jumps than with a snowboard (when you land forward anyways). another handy upside to skiing is that now when you land sideways you'll eject instead of digging 3 inches into the snow and slamming your head really hard. actually you can still do that, but the chance is minimized.