Ensure it's 100% Agave, this should be stated on the bottle. If not then it's usually a mixto which will mean a blend of agave and cane sugar. This means you're not getting 100% tequila, a true tequila should be made from 100% agave plant giving it the true agave taste.
NOM is short for "Normas Oficial Mexicana" all tequila labels are required to display a NOM. This number identifies the distillery and parent company where the tequila is made and that it meets government standards. All tequila made from 100% agave must have NOM information on the label. Multiple tequilas can be made at the same distillery and will share the same NOM, there are a handful of tequilas which have there own distillery, Don Julio being one of them (NOM 1449).
This ensures that the minimum standards have been adhered to and the label is displaying the correct information. Distilleries facilities are inspected by regulatory officers from the Tequila Regulatory Council a the standards have been met.
Hecho en Mexico (Made in Mexico)
This shows the tequila is a true product of Mexico and has been distilled and bottled in Mexico.
This is the type of tequila your getting and will make a big difference in finding one to your particular taste.
There are more than 500 brands of tequila which are produced by only about 70/80 distillers so it's important to recognise which brand of tequila you're buying.