Most waitstaff are honest, but if you are really very concerned about tip fraud or someone just mistyping a number and wanting to be able to address it:

1) Draw a dash to the left of the tip and total lines, and there is less room for adding a "1". Bonus: use your own pen with an less common ink color like turquoise or purple.

----5.00
---15.00
---20.00

2) Checkdigit your tip. Leave a tip that causes the restaurant bill to add to a funny number. Examples: on a $60.00 bill, make the bill total $70.07; for a $70.00 bill, it would be $82.68 - the tip you add makes the first digit in the total match the last digit in the bill total. Or make the total a numerical palindrome ($82.28), or something else that rocks your world.

This will stick out on your statement, and if you have to make a chargeback claim with the credit issuer, you can tell them what you do, and why this charge is not like all the rest of your historical restaurant bills.

I don't bother with either of these, but I haven't run into waitstaff that are crooked with the tip. If you are in a squirrelly area, then your mileage may vary.

[There's room for a thread in Omni for all the other ways that employee fraud occurs in restaurants/bars, besides finagling the tip]