Don't take this as an encouragement to do the work without a permit, you should always get a permit that is the law in most states, and if you get caught you will get fined, or pay the consequences if someone gets hurt, or cause any property damage as a result of faulty work, etc, not to mention it's always good to have work inspected to make sure everything is done right and most importantly to be within the code compliance, etc.

Other than that how people get caught, for simple reasons i.e someone calls on you and the city will shut you down and issue a fine. But usually, in NY, neighbors don't call on neighbors unless you or your workers will piss someone off, in most cases that happens, if the contractors make noise during early or late hours, if construction debris flying all over the place, if contractors block driveways and neighbors can't get in or out, etc. or if something bad happens, a fire, etc.

That said Brooklyn is a unique place, just getting a minor renovation permit could delay the job for up to 6 months give or take, and that is one of the reasons some companies will do work without permits if the owners OK with it, especially now with the virus, getting something done by the building department is like most NYers would say "fuggedaboutit" (originally from there) and I haven't met anybody yet who got a permit for any major interior renovation and I'm talking about complete interior remodeling jobs anywhere from 250k and up.

There are some companies who will do the work without permits and they know the "ins and outs" how to do that without getting caught, but if it's a major remodeling on the exterior of the house or it involves an addition, etc, not many will do the work because they will get caught.

Good luck and try to get a permit, you will sleep better at night.