Check for co-planar objects
This is a RhinoScript for Rhino for Windows only.
Unzip and save the CheckCoplanar.rvb file to your hard drive, then drag and drop it onto a Rhino window. This will add a alias "Coplanar" that will act much like a command when typed or added to a button.
Workflow - select two planar objects and the command line will report on whether the two objects are in the same plane.