Its a completely reactive rather than proactive approach. If someone gets caught, they will come down hard on them (like the Storm and Bulldogs) to preserve the integrity of the game and give the impression that they enforce the salary cap. But they're not going proactively look for cheating, even though they know it happens.
All you need to do is listen to any ex-player, administrator or Jurno. They all say teams cheat the cap, they all know and constantly even joke about it. Almost everyone has a story about dodgy deals and theres been so much evidence over the years.
The NRL is one of the only competitions in the world that has a salary cap with zero transparency. Its designed to be cheated. Theres a million ways around it.
Any competition needs to have one or the other. Either a salary cap with complete transparency of player earnings (US sport) or if there is no transparency on player salaries it needs to be a free market with no salary cap.