I read everywhere that one should only drink bottled water. But if we're bar-hopping, how do I ensure that cocktails are safe to drink?

The cocktail is probably safe, but I would be concerned about the ice.

Relax. If you are going to nice cafés & bars such as hotel etc nothing to worry about.

Stewbear is right.

BTW .... it is 2006 and Pancho Villa is dead.

Unless you fall into some grimy hole-in-the-wall dive, there is no need to worry about the ice. Getting sick from the ice is an 'urban legend' that just keeps getting repeated. It is made from purified water in any place you are likely to be.
Been involved with Mexico since 1955. Trust me on this.
Norm

Thanks everyone for the quick reply. I just don't want a repeat of a not-so-pleasant experience in another country I visited. You've all been very reassuring!