When the user asks to paste data from the Clipboard, you
should request a specific format and provide a callback method which will be
called with the actual data. For instance: 
refClipboard->request_contents("example_custom_target",
    sigc::mem_fun(*this, &ExampleWindow::on_clipboard_received) );
Here is an example callback method:
void ExampleWindow::on_clipboard_received(
    const Gtk::SelectionData& selection_data)
{
  Glib::ustring clipboard_data = selection_data.get_data_as_string();
  //Do something with the pasted data.
}
To find out what targets are currently available on the
Clipboard for pasting, call the
request_targets() method, specifying a method to be called
with the information. For instance:
refClipboard->request_targets( sigc::mem_fun(*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.
}