When in doubt, go for the taco salad with no beans. And if possible, get them to put the condiments (sour cream, guacamole, etc.) on the side. That way, you're in control.

Anytime I end up at a fast food place, I'll generally order a cheeseburger and then pitch the bun. But watch out for ketchup and onions! Some places love to pile them on!