I'm not sure that the Treasury needs to; all of the data is easily publicly available. Let's take it from the top.
Significant Bilateral Trade Surplus
China has maintained a significant bilateral trade surplus with the United States for most of the last 10 years. See chart here. Now, there are definitely a few dips in here, and I don't know what a "significant" trade surplus is, but if I had to guess, that looks rather significant to me.
Material Current Account Surplus
China has kept a material current account surplus in 19 of the last 20 quarters. See chart here. Easily checked.
Country engaged in persistent one-sided intervention in the foreign exchange market
This was the big news headline that prompted the call to declare China a manipulator. For those reading this in the future (hi, future guy!), see articles like this one. China often changes the value of the yuan in order to drive its economy in the direction it wants, and it does so directly, rather than how the US would achieve something like this indirectly through quantitative easing, for example.
The real reason people are scratching their heads is: why now? As you can see from my data, none of this is new. China has been a currency manipulator for most of the last 5 years, and further beyond as well.