Leveling and trueing you mill

Ok, I'm slowly progressing w/ the design of my mill/router. I'm
wondering however how a person can accuratly level[ when I say level
I mean get it perfectly flat/true, or at least close to perfectly] a
mill's deck as you build it? I can get it straight[ or straight
enough anyway] with high tension on super thin fishing line, however
that only works say along the x axis or the y axis.. but what about
the north end being co-planer with the south end of the y axis, etc?
How do you ppl get it level two ways [ N/S and E/W]?

I've used laser level's in the past and wasn't impressed. My father
and I are building a plane and when we went to set the dihedral in
the wings [ the tips are higher then the center of the wing] we
first used a lazer level.. then the old tried and true of a water
level and found the wings 3/8" [ memory there] out of level. W/ the
water level we go it dead straight.. I've heard people say a
machinist's straight edge.. but.. I don't follow how that would
work.. maybe I'm just a simpleton.

Opinions welcome...
Web links REALLY welcome..

Jarrett Johnson