I doubt this is fully verifiable to the public, but there are many conceivable ways, some obvious (and verifiable), others speculative.
The obvious way is via satellite monitoring. Google Maps includes satellite images of the whole world, and it's obvious that the US military will have even better capabilities. They can, therefore, see the Russian military units in action. This kind of reconnaissance can be seen in some news articles, e.g. this.
The other way is espionage. You can be sure the US (and Ukraine/Russia) are constantly attempting to spy on each other. It's possible that one or several of the Russian soldiers fighting in Ukraine right now are secretly US spies, and they relayed information about the upcoming attack.
Is it possible that the Russians prepared them in advance to ensure their neutrality?
Given that Russia did not inform China prior to attacking, this is extraordinarily unlikely.