Some good options for low-carb fast food are bunless burgers, salads and Chipotle bowls. In-N-Out, Five Guys, and Carl's Jr. will all replace their traditional buns with lettuce at no extra cost, making it easy to limit carbohydrates.
Aside from the bun, condiments and toppings can be a major source of carbohydrates. Avoid ketchup, barbecue sauce and relish, and don't get a side of fries.
A Chipotle bowl is very low-carb if rice, beans and corn are omitted. Jimmy John's offers anything on the menu as an Unwich, which replaces the bread or tortilla with low-carb lettuce.
For salads, avoid any sweet sauces or vinaigrettes. They're both packed with sugar, a major carbohydrate.