It isn’t quite that straight forward.
The traditional recipe for tortillas uses pure corn, and there are still a lot of pure corn tortillas to be had in Mexico. Mostly that is what you will find in local restaurants. If you buy frozen tortillas in a North American supermarket though, it is much more likely to be made of (or with) wheat.
Even in Mexico, not all tortillas are pure corn. Firstly, there is the wheat tortilla, identified by its colour. It is nearly white, where as a corn tortilla is yellowish, and usually has more texture. Large, hand-made tortillas are generally safe. Everyone I spoke to in Mexico was very sure that a hand-made tortilla is pure corn, and that’s the end of it. This may not be true in northern Mexico, I have not had the chance to look into this.
Tortillas bought in Mexican supermarkets may be wheat-free, but they are just as likely to have wheat in the ingredients. You must check every time. If you eat at a food stall, ask where they get their tortillas from. For this, you must know some Spanish, so if you don’t know any Spanish, look for a food stall that uses large tortillas that do not look machine made. If you see the owner making their own (they squeeze a ball of maize dough in a big press, then slap it on a hot grill) then you are safe (at least in central and southern Mexico, again, I have not had the chance to verify northern Mexico).
Of course “tacos” bough in the USA or Canada are pure corn. These are hard discs formed into a U-shape, which can be stuffed with meat and vegetables. They are not Mexican at all, but they are wheat-free!