First of all, you need to know your store’s coupon policy regarding doubling. This information can likely either be found at the store’s website, or at Customer Service. Most stores that double coupons (aside from during special promotions) only double coupons with face value (i.e. amount off) up to $0.99.
Some stores will only double up to $1; i.e. if the coupon is for $0.75 off, they will increase the discount to $1. Other stores fully double any coupon $0.99 or less, so if the coupon is for $0.75 cents off, they will take off $1.50. (Usually, the coupon will only double up to the cost of the item. However, differences in store policy, current promotions, and sometimes unknown pricing anomalies, might cause the coupon to double up to a value more than the purchase price of the item, which will end up giving you some discount ("overage") on other items you are purchasing. If there are not sufficient other items in your transaction, this will result in a negative due balance, which will necessitate a manual adjustment of your bill).
Secondly, certain coupons will not double regardless of your store’s policy. There are two general reasons this might happen: either the computer prevents it from doubling automatically, or the cashier manually prevents it from doubling. Some coupons have language prohibiting doubling, such as, "Do not double," or "Not subject to doubling." For these, to determine if it might double anyway, you can look at the barcode number.
If the number starts with a 5, the coupon will double automatically unless the cashier reads the coupon and manually presses a button to prevent the coupon from doubling. If this is the case, it is useful to familiarize yourself with the habits of different cashiers. Often with experience you will be able to distinguish the cashiers who will always press the do not double button and those will will not bother with it. If the barcode number starts with a 9, the coupon automatically will not double, regardless of whether there is language on the coupon prohibiting doubling. (These 12-digit codes are only available on the older, UPC-style barcodes.
Coupons today increasingly have a second barcode, which is a newer technology and does not include a printed number. Today, most coupons with this new barcode also contain the old barcode. However, some coupons today only contain the newer-style barcodes. For these coupons, there is not currently a known way of determining whether or not the coupon will automatically not double.)
Occasionally, you may encounter a coupon with no language prohibiting doubling, maybe the barcode starts with a 5 or maybe there is only the newer barcode without a number, and the coupon does not double. For this situation, you will have to decide whether or not it is worth it to raise the issue with the cashier, who will most likely call a manager, who may or may not manually give you a discount for the doubled value of the coupon.