This kind of stuff is trivial for the IRS to find if they wanted to. Just crosscheck revenue, purchases and wage costs and such. And when stuff is off balance, audit. Here in the Netherlands they even look at how much mayonaise and water a restaurant uses to estimate revenue. Same as number of cans of hair product for hairdressers.

And if a company is found at fault they force them to switch to predominantly taking card payments instead of cash to make it even more transparent for the IRS.

Apparently they also estimate people's net worth by looking at registered cars, real estate and the length of you yaght.. and this is pretty accurate to determine if an audit is in order.