While the IRS has been treating crypto as property, the recent tax bill created a more stringent definition. Trades resulting in changes in fiat currency have long been treated as capital gains subject to taxation. Now, it's more complicated. You have to declare transaction which created a change in the holding of a particular crypto, including forks. The IRS has data on Coinbase traders who traded at least $US 20,000 in 2013-15.

For more information, please see http://www.nextgov.com/analytics-data/2018/04/if-you-traded-crypto-coinbase-irs-might-be-coming-you/147291/.