partial class Examples
{
  public static Result IsPlanarSurfaceInPlane(RhinoDoc doc)
  {
    ObjRef obj_ref;
    var rc = RhinoGet.GetOneObject("select surface", true, ObjectType.Surface, out obj_ref);
    if (rc != Result.Success)
      return rc;
    var surface = obj_ref.Surface();

    Point3d[] corners;
    rc = RhinoGet.GetRectangle(out corners);
    if (rc != Result.Success)
      return rc;

    var plane = new Plane(corners[0], corners[1], corners[2]);

    var is_or_isnt = " not ";
    if (IsSurfaceInPlane(surface, plane, doc.ModelAbsoluteTolerance))
      is_or_isnt = "";

    RhinoApp.WriteLine("Surface is{0} in plane.", is_or_isnt);
    return Result.Success;
  }

  private static bool IsSurfaceInPlane(Surface surface, Plane plane, double tolerance)
  {
    if (!surface.IsPlanar(tolerance))
      return false;

    var bbox = surface.GetBoundingBox(true);
    return bbox.GetCorners().All(c => System.Math.Abs(plane.DistanceTo(c)) <= tolerance);
  }
}

Partial Friend Class Examples
  Public Shared Function IsPlanarSurfaceInPlane(ByVal doc As RhinoDoc) As Result
	Dim obj_ref As ObjRef = Nothing
	Dim rc = RhinoGet.GetOneObject("select surface", True, ObjectType.Surface, obj_ref)
	If rc IsNot Result.Success Then
	  Return rc
	End If
	Dim surface = obj_ref.Surface()

	Dim corners() As Point3d = Nothing
	rc = RhinoGet.GetRectangle(corners)
	If rc IsNot Result.Success Then
	  Return rc
	End If

	Dim plane = New Plane(corners(0), corners(1), corners(2))

	Dim is_or_isnt = " not "
	If IsSurfaceInPlane(surface, plane, doc.ModelAbsoluteTolerance) Then
	  is_or_isnt = ""
	End If

	RhinoApp.WriteLine("Surface is{0} in plane.", is_or_isnt)
	Return Result.Success
  End Function

  Private Shared Function IsSurfaceInPlane(ByVal surface As Surface, ByVal plane As Plane, ByVal tolerance As Double) As Boolean
	If Not surface.IsPlanar(tolerance) Then
	  Return False
	End If

	Dim bbox = surface.GetBoundingBox(True)
	Return bbox.GetCorners().All(Function(c) System.Math.Abs(plane.DistanceTo(c)) <= tolerance)
  End Function
End Class

import Rhino
from Rhino.Geometry import *
import rhinoscriptsyntax as rs
from scriptcontext import doc
import math

def RunCommand():
    surface_id = rs.GetSurfaceObject()[0]
    if surface_id == None:
        return
    surface = rs.coercesurface(surface_id)

    corners = rs.GetRectangle()
    if corners == None:
        return

    plane = Plane(corners[0], corners[1], corners[2])

    is_or_isnt = "" if IsSurfaceInPlane(surface, plane, doc.ModelAbsoluteTolerance) else " not "
    print "Surface is{0} in plane.".format(is_or_isnt)

def IsSurfaceInPlane(surface, plane, tolerance):
    if not surface.IsPlanar(tolerance):
        return False

    bbox = surface.GetBoundingBox(True)
    rc = True
    for corner in bbox.GetCorners():
        if math.fabs(plane.DistanceTo(corner)) > tolerance:
            rc = False
            break

    return rc

if __name__ == "__main__":
    RunCommand()