Yes, a code sample in the MSDN website gives details on how to identify the control that you are using.
CMFCToolBarComboBoxButton\* pSrcCombo = NULL;
CObList listButtons;
if (CMFCToolBar::GetCommandButtons(ID\_EDT\_FIND\_NDC, listButtons) > 0)
{
for (POSITION posCombo = listButtons.GetHeadPosition();
pSrcCombo == NULL && posCombo != NULL;)
{
CMFCToolBarComboBoxButton\* pCombo = DYNAMIC\_DOWNCAST(CMFCToolBarComboBoxButton, listButtons.GetNext(posCombo));
if (pCombo != NULL && CMFCToolBar::IsLastCommandFromButton(pCombo))
{
pSrcCombo = pCombo;
}
}
}