At the scale of a McDonalds ad campaign that seems easy to measure: go to the nearest Target parking lot and ask as many people as you can two questions: "are you vegetarian" and "did you see {description of ad here}". Then compare how which percentage of vegetarians saw the ad compared to the percentage of non-vegetarians saw the ad. There are even companies specialized in market research that will do this in a more representative and statistically rigorous way