The Real Estate (Regulation and Development) Act, 2016, and each state's authority under it. Most new projects must register with RERA.
Guide
Three numbers describe the same flat and only one of them is floor you can stand on. How they differ, and which to ask for first.
Article
A registration number is only worth what the lookup says it is.
Search the project on the state RERA website before paying anything. The registration shows the approved plan and the completion date the builder promised. Also search the site's complaints and orders for the builder's name.