partial class Examples
{
  private static bool IsBrepBox(Rhino.Geometry.Brep brep)
  {
    const double zero_tolerance = 1.0e-6; // or whatever
    bool rc = brep.IsSolid;
    if( rc )
      rc = brep.Faces.Count == 6;

    var N = new Rhino.Geometry.Vector3d[6];
    for (int i = 0; rc && i < 6; i++)
    {
      Rhino.Geometry.Plane plane;
      rc = brep.Faces[i].TryGetPlane(out plane, zero_tolerance);
      if( rc )
      {
        N[i] = plane.ZAxis;
        N[i].Unitize();
      }
    }

    for (int i = 0; rc && i < 6; i++)
    {
      int count = 0;
      for (int j = 0; rc && j < 6; j++)
      {
        double dot = Math.Abs(N[i] * N[j]);
        if (dot <= zero_tolerance)
          continue;
        if (Math.Abs(dot - 1.0) <= zero_tolerance)
          count++;
        else
          rc = false;
      }

      if (rc)
      {
        if (2 != count)
          rc = false;
      }
    }
    return rc;
  }

  public static Rhino.Commands.Result IsBrepBox(Rhino.RhinoDoc doc)
  {
    Rhino.DocObjects.ObjRef obj_ref;
    var rc = Rhino.Input.RhinoGet.GetOneObject("Select Brep", true, Rhino.DocObjects.ObjectType.Brep, out obj_ref);
    if (rc == Rhino.Commands.Result.Success)
    {
      var brep = obj_ref.Brep();
      if (brep != null)
      {
        Rhino.RhinoApp.WriteLine(IsBrepBox(brep) ? "Yes it is a box" : "No it is not a box");
      }
    }
    return rc;
  }
}

Partial Friend Class Examples
  Private Shared Function IsBrepBox(ByVal brep As Rhino.Geometry.Brep) As Boolean
	Const zero_tolerance As Double = 1.0e-6 ' or whatever
	Dim rc As Boolean = brep.IsSolid
	If rc Then
	  rc = brep.Faces.Count = 6
	End If

	Dim N = New Rhino.Geometry.Vector3d(5){}
	Dim i As Integer = 0
	Do While rc AndAlso i < 6
	  Dim plane As Rhino.Geometry.Plane = Nothing
	  rc = brep.Faces(i).TryGetPlane(plane, zero_tolerance)
	  If rc Then
		N(i) = plane.ZAxis
		N(i).Unitize()
	  End If
		i += 1
	Loop

	i = 0
	Do While rc AndAlso i < 6
	  Dim count As Integer = 0
	  Dim j As Integer = 0
	  Do While rc AndAlso j < 6
		Dim dot As Double = Math.Abs(N(i) * N(j))
		If dot <= zero_tolerance Then
		  j += 1
		  Continue Do
		End If
		If Math.Abs(dot - 1.0) <= zero_tolerance Then
		  count += 1
		Else
		  rc = False
		End If
		  j += 1
	  Loop

	  If rc Then
		If 2 <> count Then
		  rc = False
		End If
	  End If
		i += 1
	Loop
	Return rc
  End Function

  Public Shared Function IsBrepBox(ByVal doc As Rhino.RhinoDoc) As Rhino.Commands.Result
	Dim obj_ref As Rhino.DocObjects.ObjRef = Nothing
	Dim rc = Rhino.Input.RhinoGet.GetOneObject("Select Brep", True, Rhino.DocObjects.ObjectType.Brep, obj_ref)
	If rc Is Rhino.Commands.Result.Success Then
	  Dim brep = obj_ref.Brep()
	  If brep IsNot Nothing Then
		Rhino.RhinoApp.WriteLine(If(IsBrepBox(brep), "Yes it is a box", "No it is not a box"))
	  End If
	End If
	Return rc
  End Function
End Class

import Rhino

def IsBrepBox(brep):
    zero_tolerance = 1.0e-6 #or whatever
    rc = brep.IsSolid
    if rc: rc = brep.Faces.Count == 6

    N = []
    for i in range(6):
        if not rc: break
        rc, plane = brep.Faces[i].TryGetPlane(zero_tolerance)
        if rc:
            v = plane.ZAxis
            v.Unitize()
            N.append(v)

    for i in range(6):
        count = 0
        for j in range(6):
            if not rc: break
            dot = abs(N[i] * N[j])
            if dot<=zero_tolerance: continue
            if abs(dot-1.0)<=zero_tolerance:
                count += 1
            else:
                rc = False

    if rc:
        if 2!=count: rc = False
    return rc


if __name__=="__main__":
    rc, objref = Rhino.Input.RhinoGet.GetOneObject("Select Brep", True, Rhino.DocObjects.ObjectType.Brep)
    if rc==Rhino.Commands.Result.Success:
        brep = objref.Brep()
        if brep:
            if IsBrepBox(brep): print "Yes it is a box"
            else: print "No it is not a box"