BSNL Broadband usage can be checked online using this link. This links requires your BSNL username and password.
After entering login credentials click on the Login button. This would take you to the web portal which provides the details about your broadband user account.
On clicking Service Records link available on the left hand side of web page would display the Service Records for different month. And selecting the required period radio button would display the total usage for that period.
To the total usage you need to always check the value given under Sum Traffic Excluding Night Unlimited (GB) column.