Here's the thing about Tax evasion--the only way to get caught, is to spend money without paying taxes on it. If you just have the money sitting somewhere, unless someone actually finds the money, you're 100% fine. As far as auditing the Car Wash? It would come up clean, unless Skylar is a moron. Money laundering is just about making sure you pay on the money you earn. If an auditor comes in, he's not going to check to see if you actually had 1 customer per 1 transaction, that's not the way it works. What he's looking for is total receipts, than checking it against what you claimed, to make sure that all the money you made in a given year, was put on the IRS's desk, and taxes were paid on it (Appropriately--a lot of people who get caught for "tax evasion", typically just claimed crap that they couldn't prove and or lied about).
Skylar's laundering money by essentially creating fake customers. As long as it's not absolutely absurd, she won't get caught, because, again, the thing the IRS checks for is essentially that all the money the business makes, is probably accounted for. If ghosts are buying car washes? They could really care less (Again, absurdities not withstanding.)In fact, one of the reasons service industries and food industries are used is because they don't have accounts to keep track of, so it's easy to make a bunch of ghosts and impossible for the government to prove they were ghosts (Businesses with actual accounts aren't used for laundering, usually). The only way people get caught when they do this, is by being really stupid, or attempting to spend large amounts of money that they do not launder first--because they assume the laundering will prevent anyone from looking at them.
Usually people who got put away for tax evasion, did just that. They had enormous amounts of money they didn't claim and never paid taxes on--and had clear records of themselvesspendingit.
Not saying Walt won't go to jail for Tax evasion--but just pointing out, money laundering? Is hard to catch through just accounting, usually it's tied in with other charges after the person is monitored for a long, long time.