I'm a restaraunt owner and can speak of the taxes charged to servers who collect tips. If they are audited, they will be required to report their tips as income and pay their tax. Unless something has changed in the last few years, the IRS would systematically assume the server earned 10% of the revenues he/she served that day in tips. Someone above posted 15%, but I don't know if that figure is the current one used by the IRS. I'll have to get my accountant to verify that.
In my place the servers must report themselves what they made in tips. This goes into their paycheck calculations and taxes are withheld from their checks.