I have to chime in about the creative writing in trying to avoid receipt changes. DH also likes to leave a cash tip, so he would put a dash on that line and then write the check amount on the total line. One restaurant ran up a huge tip and when challenged, blamed it on their scanning machinery. They claimed that the long dash was read as a number and their system just collects the check amount and the tip line, not the total amount. Since he started putting a big zero on the tip line, we haven't had a problem. He hands the signed check and the cash to the server, so there's no confusion.
In the OP's case, I wouldn't assume that it was a deliberate attempt at fraud, just based on the numbers. Don't worry too much - just call the credit card company and dispute the entire check. Putting in $15 for the check amount and $15 for the tip could have been the server's data entry mistake, sure, but the restaurant manager should be spotting oddities like that before closing the credit card batch and they should have come up $13 over in the drawer, which is another thing the manager should have checked. If it was a fraud attempt and the drawer count was okay, then the server took $13 cash out to make it balance, which is theft.
If the restaurant is so poorly run that two people can overlook an error, how will they prevent real fraud from happening? If it's a chain restaurant, contact the corporation's customer server. Otherwise, the chargeback will help alert the owner that there's an issue.
Make sure you have your copy of the receipt handy when you call the credit card company.
I'm starting to agree that tipping should be eliminated, which will make the meals more expensive, but it will eliminate the potential for fraud. Maybe some big chain restaurant will give this a trial run someday.