There is no completely foolproof way to measure the impact of a promotional campaign. This is partly because even a successful campaign might not lead to immediate sales. A campaign might, instead, lead to people being more aware of your brand or firm, causing them to buy from you later on.
That said, there are ways to try to measure the impact of an advertising campaign/promotion. You can measure the amount of foot traffic or the levels of sales in your store before, during, and after the ad campaign. You can also try ways to determine whether the ads are actually reaching people. For example, you can put a coupon in the ad and have them bring it in. You could code the coupons differently for different media outlets as a way of determining where the people are seeing the ads. By doing things like these, you can attempt to estimate the effectiveness of your promotion.