refClipboard->request_contents("example_custom_target",  SigC::slot(*this, &ExampleWindow::on_clipboard_received) );

void ExampleWindow::on_clipboard_received(const Gtk::SelectionData& selection_data)
{
  Glib::ustring clipboard_data = selection_data.get_data_as_text();
  //Do something with the pasted data.
}  

To find out what targets are currently available on the clipboard for pasting, call the request_methods() method, specifying a method to be called with the information. For instance:

refClipboard->request_targets( SigC::slot(*this, &ExampleWindow::on_clipboard_received_targets) );

In your callback, compare the list of available targets with those that your application supports for pasting. You could enable or disable a Paste menu item, depending on whether pasting is currently possible. For instance:

void ExampleWindow::on_clipboard_received_targets(const Gtk::SelectionData& selection_data)
{
  bool bPasteIsPossible = false;

  //Get the list of available clipboard targets:
  typedef std::list<Glib::ustring> type_listTargets;
  type_listTargets targets = selection_data.get_targets();

  //and see if one is suitable:
  for(type_listTargets::const_iterator iter = targets.begin(); iter != targets.end(); ++iter)
  {
    if(*iter == "example_custom_target")
      bPasteIsPossible = true;
  }

  //Do something, depending on whether bPasteIsPossible is true.                                          
}