Enforcement is addressed by Section 90.4, which covers the responsibilities of the authority having jurisdiction (AHJ). It is the responsibility of the AHJ to make interpretations of the rules and to approve installations and equipment.

This is a very broad requirement. It essentially requires the AHJ to approve everything, because approved is defined in Article 100 as "acceptable to the authority having jurisdiction," and everything installed under the NEC is acceptable only if approved, according to Section 110.2.

The issue of who is the AHJ is not specifically addressed in Article 90.4. It is somewhat addressed in the definition of AHJ in Article 100, which says that it's essentially a representative for whoever or whatever entity adopts the code.

Often there is more than one AHJ. For example, where OSHA has jurisdiction, it will have some authority over aspects of an electrical installation, as will a local building official or inspector, a fire marshal and perhaps an agent of an insurance company.