Most Mexican food is not in itself terribly spicy – the fire comes from the red and green salsa supplied as condiments on the table, and the salsa can be a good guide to the quality of a restaurant. A place with a superior salsa on the table will probably serve up some decent food, whereas a place that takes no pride in its salsa is likely to treat its food in the same manner. To a certain extent you can tell from the presentation: a place that has grubby, rarely changed salsa dishes probably just refills them from a supermarket-bought can, and will not take the same pride in its food as a casero (home-cooking) restaurant that proudly puts its own salsa on the table in a nice bowl. Usually you get a red salsa and a green one, and sometimes bottled hot-sauce condiments too.
Nowadays, the red bowl may contain a raw, California-style salsa: tomato, onion, chile and cilantro (coriander leaves) finely chopped together. More common, though, are the traditional cooked salsas: either green or red, and relatively mild (though start eating with caution, just in case). The recipes are – of course – closely guarded secrets, but the basic ingredients are tomato (the verdant Mexican tomatillo in green versions), onion and one or more of the hundreds of varieties of chile.