Determining widget type

[Phil Wolff <adiabat centurylink net>]

I want to modify the primary message widget so I can select and copy the 
text.

    Gtk::Box* pBox = pDialog->get_content_area ();
    std::vector<Gtk::Widget* > v = pBox->get_children ();
    Glib::ustring str = ((Gtk::Label*)v[0])->get_label ();

This fails: Gtk-CRITICAL: 08:51:12.080: gtk_label_get_label: assertion 
'GTK_IS_LABEL (label)' failed

So the primary message widget isn't a Gtk::Label. How do I determine 
what it is?