CComboBox::GetDroppedState
Call the GetDroppedState member function to determine whether the list box of a drop-down combo box is visible (dropped down).
BOOL GetDroppedState( ) const;
Nonzero if the list box is visible; otherwise 0.
// Show the dropdown list box if it is not already dropped.
if (!m_pComboBox->GetDroppedState())
m_pComboBox->ShowDropDown(TRUE);
Header: afxwin.h