Mexican can be a great cuisine for vegans with a high numbers of dishes having a bean option, which adds some needed protein a fibre to your meal. It's important to keep an eye out for some common animal based ingedients that can sneak into even technically vegetarian dishes.
Queso, or cheese, is a common ingredient in most Mexican meals especially in Westernised menus. Even in menu items that don't explicitly use cheese, you can often find small amounts of cheese in the sauce to give it a creamy texture, so it's a good idea to ask whether there are any animal products such as queso in vegetable or bean dishes. Try asking '¿Esto contiene queso?' if the waitstaff speak mainly Spanish. It is relatively easy for kitchens to substitute vegan alternatives into sauces to give a similar texture and taste to cheesy dishes. You can also ask to swap out cheesy sauces in your dishes for vegan options such as guacamole.
Some Mexican restaurant will use animal fats such as lard or butter in their refried beans. This can be an issue for vegans, so you need to specify that you want dishes cooked in vegetable oils such as sunflower oil. In Spanish, this is know as 'aceite vegetal'. Again, many restaurants will be able to make this substition if they know it is an issue, but dishes that have been prepared in bulk may include animal-based oils.
Another common ingredient in Mexican restaurants is sour cream, especially in nachos and quesadillas. You can try to substitute these ingredients but it can be difficult as many vegan substitutes have a higher water content and can make your crispy dishes soggy and hard to handle. In many cases it can be easier to simply ask for the sour cream on the side so that you get a dishes with the right texture. Try asking for 'Crema agria en el lado, por favor' to ensure that the sour cream is served on the side.
There are a range of easy and naturally vegan foods on offer at most Mexican restaurants such as black bean soups, making this style of cuisine great for vegans. If you are unsure of whether your Mexican dishes contains animal products, then you should ask the waitstaff for some clarification. This will allow you to eat your food without any worries about the ingredients.