If you closely review your card statements and need to track receipts for expenses then you may have experienced this first hand. The scam is that the wait staff adjusts the tip and the total amount of your check by making small "additions" to your writing after you leave. This has happened to me more than once. I had someone at a restaurant in Fort Worth do this for an additional 50 cents. Seriously.What I now do is to write out the amount below the total and above the signature. I actually don't know if it will help but I have not had to deal with another instance of this type of fraud since I started doing this. It will be pretty clear looking at the receipt if the numbers don't match the words. And, in my opinion, more defensible when you need to call and refute an overcharge. Another thing I do is to take a picture of the completed and signed receipt.
A while back I wrote about tips on protecting your payment card when making retail purchases. Check them out and let me know if you find them helpful.