I can tell you how!
first, roll the tequila around the glass...the drip pattern back to the bottom will expose the agave nectar.
solid, substantial streaks indicate the presence of nectar, and therefore the quality of the spirit.
next, place your nose at the bottom edge of the rim and inhale. gain a sense of the bouquet and flavors.
while your lungs are at low capacity, deposit a small (or large) quantity of tequila upon the tip of your tongue.
imbibe the sip immediately, and draw in air whilst you do so. the character of the spirit should be revealed.
common brands such as Cuervo are for good for degreasing engines and killing brain cells.
1800/Patron is sorta approaching okay.
I have lived in Mexico for almost 5 years, and it is my job to know this...kgraciasalud!