How often are we talking about with "once in a while"?
If we are talking about once every 4-6 weeks, Dude just have the salad and enjoy the heck out of it. See how you feel afterwards, and determine if the salad pleasure is worth the consequence. Maybe dose up on the O3 acids afterwards to help counterbalance.
For the vast majority of restaurants, I would bet on the dressings/marinades/grill prep to be done with canola or other processed vegetable oil. That is the reality of the industry.
If you are thinking of once per week, then that is a bit more industrial fat than I would be comfortable with. As you transition to more primal eating, you will be able to isolate how specific foods affect you, and then eliminate those that cause negative side effects.