I have seen many queries asking how to check the usage details. Most probably it is the most sought out question in this forum. Here is my small contribution towards this topic.
1. Go to data.bsnl.in, enter your 14 digit portal ID, under postpaid services check your usage details.
2. SMS and email alerts. Go to bbusage.bsnl.in and register your account. Please give ample time before you get alerts.
3. Install a third party bandwidth meter. Shaplus bandwidth meter will be a good choice.
4. Hyderabad users, please check here - ww.selfcare.sdc.bsnl.co.in
These are 4 different source to check your usage details. There are many tools which provide access through their interface. One example is How to check BSNL Broadband usage