You could create a custom module and an event observer hooked to the cataloginventory_stock_item_save_before
event then you can validate if the qty will be saved as negative and perform some action.
Configuration:
Vendor/Module/etc/config.xml
<global>
...
<events>
<cataloginventory_stock_item_save_before>
<observers>
<vendor_module_stock_item_save_before>
<class>vemdor_module/observer</class>
<method>stockItemSaveBefore</method>
</vendor_module_stock_item_save_before>
</observers>
</cataloginventory_stock_item_save_before>
</events>
</global>
...
Observer:
Vendor/Module/Model/Observer.php
class Vendor_Module_Model_Observer
{
public function stockItemSaveBefore(Varien_Event_Observer $observer)
{
$item = $observer->getEvent()->getItem();
// Perform some action ...
}
}
I don't know what do you want to do with that information. You may want to send an email, save a log or just keep the qty in 0, regardless the action you'll take this is the way to do it.