So here's a couple tips about receipts at restaurants that could keep you from getting ripped off.
I was eating at Nashville's Harley Davidson Cafe once. My bill came to about 12 bucks. I threw three singles down on the table as a tip and went up to the cashier to pay the rest on my debit card. On the line where it said "tip", I left it blank (because I already left the tip). That's when the really helpful cashier told me this tip. "If you paid the tip in cash, write "0" on the tip line of the receipt. If you don't, a server could later fill that part in and it would get added to the bill on your debit card." That was a great tip. I never thought of that.
Here's an idea that could save you from getting scammed even when you're tipping on the receipt. Always take a copy of the receipt to prove what you wrote down. Restaurant insiders on Reddit have admitted that's the only way to protect yourselves in situations like this:
Redditor u/ onekate said:
I had a dishonest friend who I learned would change every tip that wasn’t 20% to at least that. Every single one. Never got caught. Because of him, I write the total amount of my bill including tip below the numeric total and above the signature line like one would write it on a check. Every time.
To be fair, if you've had good service, 20% is a proper tip. But if it wasn't worth this standard tip, don't let them round up!