Not usually:
"With the exception of some business accounts, Coinbase does not provide 1099 forms. Coinbase does however provide a specialized Cost Basis for Taxes report which will help with filing your taxes." [1]
After this hypothetical transaction you will likely owe capital gains tax. It would be hard to hide any significant amount from the IRS if you actually want to convert it to USD, for example by transferring to a bank account [2]. Regardless, the IRS is definitely paying attention and going after back taxes on large exchange accounts [3].
[1] https://support.coinbase.com/customer/en/portal/articles/149...
In the US, presumably? It's ever-so-slightly different from other investments. AFAIK it's a real grey area regarding mining. If you mined the coins, maybe those were taxable income then (offset by big capital expenses?), at the market value at that time. I think that's where the differences might end. Now that you have them, plain old capital gains tax likely applies.
> how will the IRS ever find out? ... What if you just keep the gains and keep quiet?
You end up accepting some risk that you don't end up being investigated. Very much of the taxes that you pay that are not part of withholding operate this way. Be honest and have nothing to fear or risk an audit.
You risk paying much more than the usual tax if you get caught, and that's if you simply "forget to declare". If it looks like that you've taken explicit steps to hide those gains, then criminal charges may get pressed. IIRC they get some 2000 criminal convictions a year for tax evasion.