You can always check your available Jio balance by opening the MyJio app on your smartphone but there is another way to check it. And that is by giving a missed call to 1299.
Follow below steps :-
Step 1. Go to your phone dialer and place a missed call to the number 1299
Step 2. After ringing for few seconds, the call will get automatically disconnected
Step 3. You'll immediately get an SMS explaining the plan (with validity & available MB) activated on your Jio number like following screenshot
That's it. It's that simple.