create a 'Hitpart' eventHandler on a building with doors and add something like
{
_array = _this select 0;
_build = _array select 0;
_selection = (_array select 5) select 0;
_hitSelPos = _build selectionPosition _selection;
hint (str(_hitSelPos));
};
as the code to execute when the EH fires.
notice how the hint returns [0,0,0] which selectionPosition does when the selection is incorrect.