Just like normal pointers, you can check whether a RefPtr points to anything.

Glib::RefPtr<Gtk::TreeModel> refModel = m_TreeView.get_model();
if(refModel)
{
  int cols_count = refModel->get_n_columns();
  ...
}

But unlike normal pointers, RefPtrs are automatically initialized to null so you don't need to remember to do that yourself.