Guys level fence posts the same way.  They slap their level on 1 side at a time & trim 1 axis at a time.  They iteratively trim 2 axes over & over.  It's rather tedious.  The thought occurred of using a plumb bob but that doesn't work in wind.  

There is a standard post leveling tool which shows both axes simultaneously.  

The thought occurred of creating an electronic post leveler which could graph the axes & show which way to tilt it.  It might be over complicated.