As a simple thumb rule you can consider that if the project you are invested in,has a plot size of more than 500 sq. mt or the total number of apartments being constructed in the building or project is more than 8, then the project should be registered with RERA Authority.
As a consumer you can always ask the developer to provide reason in writing, why his project is not registered with the RERA Authority. If developer doesn’t reply properly, you can consider there is something fishy here. Start digging deep and good luck for your investments!!