Hi Deepak,
You can make use of Dynamic Availability check Configuration.
Materials Management, Inventory Management and Physical Inventory → Reservation → Set Up
Dynamic Availability Check (OMB1).
I guess the Standard Setting uses Checking Rule A for MB21 transaction
You may assign this checking rule for 311 Movement type and you should get a warning message whenever stock level is below the reservation quantity. You may also need to define the checking rule if you are using custom Checking Group for Availability Check.
Regards,
Niranjan