The NOM is the Official Mexican Standard of Tequila that is regulated by the Mexican Government, which contains specifications to be followed by all individuals and companies producing Tequila.
In short, the NOM ensures the protection of the Appelation of Origin, regulates production, and continually establishes standards required to ensure quality. Those companies that comply with the regulations setforth by the Tequila NOM are then certified by the TRC (Tequila Regulatory Council), and labeled as an authentic Tequila producer.
How can you tell if the tequila you are drinking is authentic? Look for the NOM number on either the front or back label on the bottle. The NOM number is a unique, government regulated distillery number. Many distilleries produce more than one brand of tequila, and some brands are exported, and some are only available in Mexico. Those products containing the same NOM # are produced from the same distillery.
If you have a favorite Tequila, try sampling another brand from the same distillery, as they will most likely be similar in the way they are produced.