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.
3. Customer query: Instruct your staff to simply ask customers why they're there and how they heard about you. Tally the answers, and you'll see the effect of your advertising.