The same way cash transactions are trackable, which is to say they are not. In either event you can choose to evade the taxes but it's probably not a wise bet.
They aren't trackable, but when you pay cash for a $90,000 car and a $350,000 boat the fact that you've clearly under reported something is observable and to my knowledge sufficient information to prosecute for tax evasion.
How would that be any different then if you paid Bitcoins for a car or a boat? Once you convert it into something tangible it's easier for the IRS to figure out.
