For a tequila to be pure it must be made only from the Blue Agave and state "100% Agave" on the label. If the label doesn't state 100% Agave then the contents will usually be a mix of a minimum 51% agave and 49% of sugar (generally cane sugar). These are more likely to leave you with a sore head in the morning, they might be ok for knocking back a few slammers but not recommended if you're looking for a tequila to sip.
Tequilas which are aren't labelled as 100% Agave are known a Mixtos.
For the label to display this the tequila must be bottled in the packing facilities controlled by the producer. Other variations include 100% de agave, 100% puro de agave or 100% puro agave.
Ordering a tequila in a bar usually results in being served with a single measure in a shot glass accompanied by salt and a slice of lemon. This is fine if your out for a good time with the aim of getting drunk and waking up feeling like you've been hit by a truck.
If you prefer to savour the taste try asking the bartender what tequila they have available, you could always mix it with a splash of bitter lemon.
Think of ordering a tequila as you would order a fine whiskey, you wouldn't order the cheapest whiskey and continue to knock them back until you fall over. Why do it with tequila!
Good tequilas are just like a good malt whiskey and should be treated as such, they should be savoured.
Bottles come in all shapes and sizes, if you're buying a bottle of tequila because it's shaped like a cactus, pistol or worm then that's all well and good but don't expect it to always taste like a quality tequila. Most tequilas come in a standard bottle, some are more distinctive such as Parton or Don Julio. It's about what's on the inside. You can pay for an expensive bottle but this doesn't always mean you're getting a good quality tequila.