Most coupons include legal wording that prohibits anyone from selling the coupons. Paying for coupons is a good sign that the coupons will be counterfeit.
All coupons usually include “fine print” that spells out restrictions, expiration, and other redemption details. Missing expiration date is a sure sign that the coupon is fraudulent.
It's illegal to photocopy or scan a coupon. Pay attention to the coupon source and to the quality of the coupon itself.
Universal Product Code (or UPC) is used by the retailers to enter the discount using the hand-scanner at checkout. All manufacturer coupons utilize UPC and many store coupons have it as well. If a store coupon doesn't have UPC it will usually have instructions for the associate on how to enter the discount.
Coupon Information Corporation is an association of consumer product manufacturers dedicated to fighting coupon fraud. If you use coupons a lot, it can be useful to periodically check with them for recent counterfeit coupon alerts.
Check for spelling errors, typos, or savings claims that seem excessive (e.g. 75% off purchase). These could be the signs that the coupon is a counterfeit.