None of these methods is foolproof. And none of them will work for all types of businesses. But try a few on your advertising efforts, and you'll get a sense of how well they're doing and whether the expenditure is giving you a satisfactory return on investment.
2. Before-and-after survey: Sometimes your advertising isn't selling some specific item; it's designed to increase traffic generally or increase market awareness of your company and its products. You can conduct market survey testing for the level of awareness of your company and its products. After your awareness campaign has run, you would then retest for awareness of your company and its products. Barring outside influences, you can attribute the difference in awareness to your advertising.