Checking items stock custom script

Hi, We can’t seem to find a custom script for item stock checking. We had made a module named “stock request” and we want to make the item check the qty and the serial number and gives warning message as soon as possible.
“Item 1” stock qty is = 3pcs
and then I used the Stock request module and inserted the “Item 1” and put the qty to 10pcs.
A message will automatically and immediately appear and warns “Not enough stock item”.

Please help us.