Quick answer: Track one thing you can watch before, during, and after the campaign. A sign can help bring in visits, but one number rarely proves every sale came from it.
Pick the result you care about
Pick one thing to watch: visits, calls, bookings, or coupon use. Don't pick a number just because it's easy to count.
Know your normal numbers first
Track the same thing before the campaign if you can. Note any sales, weather, or events that might change the numbers.
Keep tracking simple
Pick one person, one place to write it down, and one clear rule for what counts. Make sure staff knows the rule.
Read the results honestly
Compare the campaign numbers to your normal numbers. Don't claim the sign caused every single visit. Use what you learn to improve next time.