Each restaurant has their own way of preparing food, different places the food is sourced from and different ingredients that are in the foods themselves. It is important to stay educated on how a specific dish is prepared, and where the main ingredients are sourced from. If a restaurant can not tell you where they source their food, something has to be up. However, as long as you are being respectful, you should be able to get all the information you need to be a right decision,
Follow your intuition, but follow common sense too. Street tacos and fast food isn't going to have the nutrients you need long term, but it' okay once a few weeks. Moderate yourself, and try to go for fast but healthy options that are a bit more pricey, like Lemonade, Mendocino Farms or local sandwich shops.