partial class Examples
{
  public static Result DetermineNormalDirectionOfBrepFace(RhinoDoc doc)
  {
    // select a surface
    var gs = new GetObject();
    gs.SetCommandPrompt("select surface");
    gs.GeometryFilter = ObjectType.Surface;
    gs.DisablePreSelect();
    gs.SubObjectSelect = false;
    gs.Get();
    if (gs.CommandResult() != Result.Success)
      return gs.CommandResult();
    // get the selected face
    var face = gs.Object(0).Face();
    if (face == null)
      return Result.Failure;

    // pick a point on the surface.  Constain
    // picking to the face.
    var gp = new GetPoint();
    gp.SetCommandPrompt("select point on surface");
    gp.Constrain(face, false);
    gp.Get();
    if (gp.CommandResult() != Result.Success)
      return gp.CommandResult();

    // get the parameters of the point on the
    // surface that is clesest to gp.Point()
    double u, v;
    if (face.ClosestPoint(gp.Point(), out u, out v))
    {
      var direction = face.NormalAt(u, v);
      if (face.OrientationIsReversed)
        direction.Reverse();
      RhinoApp.WriteLine(
        string.Format(
          "Surface normal at uv({0:f},{1:f}) = ({2:f},{3:f},{4:f})",
          u, v, direction.X, direction.Y, direction.Z));
    }
    return Result.Success;
  }
}

import rhinoscriptsyntax as rs
import Rhino
from Rhino.Commands import Result

def RunCommand():
    # select a surface
    gs = Rhino.Input.Custom.GetObject()
    gs.SetCommandPrompt("select surface")
    gs.GeometryFilter = Rhino.DocObjects.ObjectType.Surface
    gs.DisablePreSelect()
    gs.SubObjectSelect = False
    gs.Get()
    if gs.CommandResult() != Result.Success:
        return gs.CommandResult()

    # get the selected face
    face = gs.Object(0).Face()
    if face == None:
        return

    # pick a point on the surface.  Constain
    # picking to the face.
    gp = Rhino.Input.Custom.GetPoint()
    gp.SetCommandPrompt("select point on surface")
    gp.Constrain(face, False)
    gp.Get()
    if gp.CommandResult() != Result.Success:
        return gp.CommandResult()

    # get the parameters of the point on the
    # surface that is clesest to gp.Point()
    b, u, v = face.ClosestPoint(gp.Point())
    if b:
        dir = face.NormalAt(u, v)
        if face.OrientationIsReversed:
            dir.Reverse()
        print("Surface normal at uv({0:f},{1:f}) = ({2:f},{3:f},{4:f})".format(
            u, v, dir.X, dir.Y, dir.Z))

if __name__ == "__main__":
    RunCommand()