They don’t do the verification before you join the academy.
Before your interview, you send 6 fully filled forms with your background information to the DoPT (Dept. of personnel). These 6 forms are forwarded to various agencies like the Police, IB etc. if you have been selected through the exam.
These agencies do the background verification at their own pace. In my case, I joined the academy in December 2016. The police personnel who were conducting the background check had gone to my home in around April or May. Mostly, this process takes place in the background and will be completed before your probationary period.
PS: Additional fact: The police personnel who came to my home was from the special branch.