If you obtained stock prior to 2011, and sold it, the IRS has no idea what the cost basis is, so this is voluntarily reported by the filer (and this will determine your capital gains). Of course, if you fudge this number to the point it is noticeable, you're looking at between a 20% and 100% penalty, or possible civil fraud.

Bitcoin is really no different. It doesn't matter if there's no easy way for the government to keep track of every taxpayer's BTC holdings, because if the IRS determines at a later time that you have provided inaccurate information, you're in trouble. Maybe one day exchanges will provide 1099s for sales, but until then, it's the same rules as any other unreported income.