In most jobs, it is each employee's responsibility to declare their tip income at the end of a shift, which is documented on their pay slips and taxes are deducted from their wage.

Some places pool all tips, and depending on the place, it may or may not be handled legally as income. Coffee shop tip jars as a prime example of tips not likely to be declared on income. Working any job where the employer is paying less than minimum wage due to expected tips, that's definitely being reported and taxed, though probably at a lower rate than perfectly legal.

Small dollar restaurants like Denny's probably have a spoken agreement between all the employees to declare a specific percentage of their total sales for each shift. It's always less than anyone actually makes in tips, but it is agreed upon to keep the numbers standard. That way, all the people getting tips are lying consistently, reducing the chances of a red flag being waved over any one of their heads when the IRS (or whomever they're all scared of) comes looking.