"I didn’t realize that our military knows what every aircraft in Syria is doing at all times. That’s impressive, bordering on hard-to-believe." - S. Adams.
Between AWACS and JSTARS, both of which have been prowling the skies nearby, there's very little that would go on unobserved.
Add in the frequent ELINT missions that are going on to learn more about the Russian S-400 system. Those birds are doubtlessly also gathering other data too.
Where the US fails in intel is almost always human intelligence, not electronic or observational.
We're watching the theater, not for the least reason that the Russians teach the people they supply with weapons how to use them. Watching the Russians prosecute a war gives us great insights into how their clients will fight. Even if we never fight the Russians, we might end up shooting at their clients or proxies.