Don't put any seed in the planter or keep the seed drive shut off if one has the capability.  Set the planter into the ground and drive along the field edge where one thinks the planter should be.  Leave the planter in the ground, get out and look behind the planter to see where the outside row unit is located.  Expect one will find they are 2-3' too far away because of the optical illusion of the higher toolbar versus down low where the row-unit is engaging the soil. 

Get back in the tractor without moving anything and find a u-bolt or something on the toolbar that is lined up at that moment with the field edge.  From that first toolbar reference point, look at the planter bar for another u-bolt or like widget further in on the toolbar that is the distance one needs to move closer to be on the field edge.  Drive ahead until that new toolbar reference point is lined up with the field edge.  Get out and look again.  Odds are one will be pretty darn close.  Now one has a visual reference on the planter toolbar versus field edge that is always there and never moves.