Summary: Demonstrates how to test if an object looks like a circle.

bool IsCircle( const CRhinoObject* obj )
{
  bool rc = false;
  if( obj )
  {
    // Is the object a circle?
    if( const ON_ArcCurve* arc = ON_ArcCurve::Cast(obj->Geometry()) )
    {
      if( arc->IsCircle() )
        rc = true;
    }
    // Is the object an curve that just looks like a circle?
    else if( const ON_Curve* crv = ON_Curve::Cast(obj->Geometry()) )
    {
      ON_NurbsCurve nurb;
      if( crv->GetNurbForm(nurb) )
      {
        ON_Arc arc;
        double tol = ::RhinoApp().ActiveDoc()->AbsoluteTolerance();
        if( nurb.IsArc(0, &arc, tol) && arc.IsCircle()  )
          rc = true;
      }
    }
  }
  return rc;
}

Function IsCircle(ByVal obj As IRhinoObject) As Boolean
  Dim rc As Boolean = False
  If (obj IsNot Nothing) Then
    ' Is the object a circle?
    Dim arc_crv As IOnArcCurve = OnArcCurve.ConstCast(obj.Geometry())
    If (arc_crv IsNot Nothing) Then
      rc = arc_crv.IsCircle()
    Else
      ' Is the object a curve that just looks like a circle?
      Dim crv As IOnCurve = OnCurve.ConstCast(obj.Geometry())
      If (crv IsNot Nothing) Then
        Dim plane As New OnPlane()
        Dim tol As Double = RhUtil.RhinoApp.ActiveDoc.AbsoluteTolerance()
        If (crv.IsPlanar(plane, tol)) Then
          Dim arc As New OnArc
          If (crv.IsArc(plane, arc, tol)) Then
            rc = arc.IsCircle()
          End If
        End If
      End If
    End If
  End If
  Return rc
End Function

private bool IsCircle(IRhinoObject obj)
{
  bool rc = false;
  if( obj != null )
  {
    // Is the object a circle?
    IOnArcCurve arc_crv = OnArcCurve.ConstCast(obj.Geometry());
    if(arc_crv != null)
      rc = arc_crv.IsCircle();
    else
    {
      // Is the object an curve that just looks like a circle?
      IOnCurve crv = OnCurve.ConstCast(obj.Geometry());
      if(crv != null)
      {
        OnPlane plane = new OnPlane();
        double tol = RhUtil.RhinoApp().ActiveDoc().AbsoluteTolerance();
        if(crv.IsPlanar(plane, tol))
        {
          OnArc arc = new OnArc();
          if(crv.IsArc(plane, arc, tol))
            rc = arc.IsCircle();
        }
      }
    }
  }
  return rc;
}