There's going to be no simple way to enforce it, as unlike Stocks and Bonds, most people don't hold BTC at an investment firm, so there won't be a way for exchanges or businesses to track capital gains and losses.

It will resemble the way that people had to pay taxes on Swiss bank accounts.  No one is going to report your gains/losses to the IRS, but the IRS will likely get a list of citizens who use an exchange / use BTC to purchase goods, and for an unlucky few, they will audit their tax return.

It wouldn't shock me if they tried the same grace period that they did with the Swiss accounts and say you have until a certain date to catch up on your records and pay taxes / after this date, if we detect unreported transactions, you're getting charged with tax evasion.