[jsgo]

to emodendroket's point, I recall reading an article that in the previous year (2016) that the IRS stated that very few people were claiming bitcoins in their taxes. I'm not versed on cryptocurrencies so I don't know how plausible it is that they know this, but it seems like something they're looking at now.

[neuronexmachina]

Basically, only 802 people told the IRS about their 2016 profits in CoinBase, one of the more popular exchanges. It's safe to assume that number was a tiny fraction of the actual US users.

http://fortune.com/2017/03/19/irs-bitcoin-lawsuit/

[AznHisoka]

How would the IRS know the exact number? What about people who lumped their bitcoin gains with their other capital gains?

[jsgo]

I imagine it is like anything else: they don't know, but if they do an audit or another agency is investigating you for something else and they notice, then they start investigating it.

I do think it is fair to say that there are more than 802, apparently, people in the United States in 2016 that mined/bought/sold bitcoin so they definitely see there is a problem.