It's all about the glass!
Sight- look for color, tone and the legs (thick runnels of liquid on the side of your glass)
Aroma - before you sip, swirl and inhale shortly. Are you overwhelmed with alcohol “burn” or is it mellower, with hints of oak barrels, brown spices, vanilla and maple to name a few?
Sip- How does it feel in your mouth? Thick, thin? Does it dry your palate out, or make you salivate more? Does the taste linger, or wash away quickly
There is no right way to tell you what makes a tequila good or bad, just your preference on the above. The best way to tell the difference, is to taste a lot of tequila!