There’s a fascinating yet frightening discussion over at the FatWallet Finance Forums about restaurant servers stealing their way to higher tips on credit card receipts. Servers upset by low tips that they probably deserved have been known to edit the tip line of credit card receipts to bump up their tip. For example:

See how easily a measly tip is changed to a generous one by a disgruntled server or bartender?

The easiest way to combat this illegal and downright nasty behavior is to reconcile your receipts against your credit card statements each and every month. You’ll spot restaurant wrongdoing and be able to phone it in to your credit card issuer faster than you can say “there’s a fly in my soup.”

But for the trillions of people who don’t want to go through the trouble of reconciling their receipts each month, there’s an easier way to stop tip alterations just by glancing over your credit card statement each month. It involves using checksums to add a layer of security to your tip amount. The term “checksum” normally refers to a technique used by computer systems to ensure file integrity. Here, we’ll be using checksums to ensure human integrity.

There are many checksum systems you can use when tipping, but here’s a great example that’s easy to learn and can be performed without the aid of a calculator unless you suck at math really badly.

Step 1: Look over your receipt

You’ll get ripped off far more often for food and drink overcharges than you will by spiteful servers. Check each billed item and compare it to the menu price.

Step 2: Calculate the appropriate tip

Tip as you normally do without worrying about checksums for now. For our example, let’s say you had the following bill but service was slow, so you’d like to tip about 10%.

Step 3: Apply a checksum

That $4.71 is just begging to have ten dollars added to it, and $51.87 becomes $61.87 so easily. Foil these alteration efforts with this simple checksum method.

Adjust the amount of the tip so that the numbers in the final total to the left of the decimal point add up to the right-most digit. In this case, the total has a “51” to the left of the decimal point (A). 5 + 1 = 6, so the final digit should be six. Adjust the total to $51.86 or $51.96 (B) by adding nine or subtracting one from the tip (C).

Step 4: Check your credit card statements each month

While the receipt will help you ensure no fraud was enacted upon your dining bill, you only need the statement to verify the checksum. For this example, simply locate the dining transaction, add the numbers to the left of the decimal point, and confirm that they add up to the right-most digit. If they don’t, you’ve been scammed.

This technique is not foolproof. If the scammy server had added nine dollars to the total—making it $60.86—the checksum calculation would still come back okay. But because it’s harder to turn a “51” into a “60” than a “61,” it’s unlikely your server will do this unless they’re wise to this particular checksum technique.

Step 5: Deal with the theft

If you hit a checksum that fails, dig out your copy of the receipt to confirm it doesn’t match the total on your statement. Next, do three things:

Call your credit card issuer. It should be fairly simple to get a credit for the difference between your actual bill and what you were forced to pay due to this fraud. Call the police. You were the victim of a crime, so you should report it, even if it’s just a few dollars. If the stealing server has multiple victims who report his or her behavior, the police will likely take action against the server and/or the restaurant. Hopefully a few thieving restaurant workers behind bars will set enough of an example to discourage similar actions in others. Call the restaurant (optional). At this point, you’ve likely got your money back and given all the information you can to law enforcement. You can try calling the restaurant to report the theft, but it might not do much. In the best case, the manager will recognize the server’s name on your receipt and confirm he or she has been suspected of wrongdoing. Maybe you’ll even get a free meal out of it for your trouble. Worst case, the restaurant does nothing.

The original poster in the FatWallet Finance Forum topic may be onto something when he or she says that this is probably one of the most widespread types of theft that goes unpunished. Now you have the tools to fight it. The next time you’re at a restaurant, eat, drink, be merry, and do a little extra math come tip time to help combat this rapidly growing problem.

UPDATE: Jeff B. put together a nifty Windows Mobile app for tip checksum calculations that’ll help make it easier to compute the proper tip given your level of service and checksumming method. Thanks Jeff!