You'll need to write a script and use one of several tricks to run the script at regular intervals to check your stock levels.
Something like this.
Enter Find Mode
Set Error capture [on]
Set Field [YourTable:: PercentInStock ; "< " & 0.15 ]
If [get ( Foundcount ) > 0 /* Some inventory low */ ]
You can set a script like this to run everytime the file is opened, everytime it is closed. You can use an Operating System scheduler to open a file that runs this script once a day, or you can use Install On Timer Script to run the script every so many hours ( or some other specified interval ).
You could also trigger a script that checks the level of the current item each time some one checks an item out of inventory.