AC refridgerant inspection window (on Old WagoR) can be seen in the photo below:
It is the small rectangular block below the circular part marked as 8. It is the part after pipe with green tape. It's on the smaller AC pipe. You may need to find that on newer Wagon R.
Note that, temperature of 7 or 8 (which your mechanic told) is not the sign of anything except your system has some gas. Unfortunately, even MASS don't follow company specified procedure which is follows:
1. Open all windows
2. Start the AC on full speed at coldest temperature.
3. Check the temperature of the air coming from center duct.
At 25C duct air temp should be 6 to 9 C
At 30C duct air temp should be 10 to 13 C
At 35C duct air temp should be 14 to 17 C
Did the mechanic took temp with above conditions.