How is the testing ban enforced in F1?

The regulations limit the amount of testing done to Formula 1 cars. How is this ban enforced?

In practise it does not need to be enforced. Operating a Formula 1 car in secret is is close to impossible. Operating a modern F1 car requires an army of engineers at the scene. In addition you would need to get access to tires from Pirelli and smuggle those to the location.