Open air food stands make their living selling food, and as such they take care in the quality of the product or they will be out of business in a hurry. The food is generally good and the prices better. Always choose a stand that has business. Stay away from ones that are set up for tourists. So my advice is try them, and the farther away you get from the tourist areas the better they are likely to be.
Hotels owned by major chains and corporations sometimes have an internal purification system.
These are advertised and unless the hotel tells you this up front, drink bottled water. Keep a bottle of purified water in your room and with you when you are traveling. The water in good restaurants is purified, yes, even the ice. You can ask for purified water and they will sell it to you, but you just paid for the same water you would have been served anyway. This applies to the tourist areas in Mexico, however, when off the beaten path you should use caution.