Both are agave spirits, but they're not interchangeable — and Cancún's resort bars often blur the line more than they should.
The core difference
Tequila is made specifically from blue agave, mostly in and around Jalisco state, and is typically produced in industrial ovens or autoclaves. Mezcal can be made from many agave varieties across several Mexican states (Oaxaca is the best-known), and is traditionally roasted in earthen pits, which gives it a characteristic smokiness tequila doesn't have.
How to actually taste it
Skip the salt-and-lime routine for anything you're trying to actually taste — that ritual exists mainly to mask the bite of cheap, mixto-grade tequila. A good tequila or mezcal is meant to be sipped slowly at room temperature.
What to order
Ask for "100% agave" tequila specifically — mixto tequila (which can legally contain up to 49% other sugars) is what tends to cause the worst hangovers and the least flavor. For mezcal, ask what agave variety (espadín is the most common and approachable) and region it's from; a good bar or shop will know.