Every country puts out an official gazette with abundant regulatory and statistical information. Of course you'd be foolish to rely on all these at face value, but it's an excellent starting point for assessing the economic activity of any given country. You can then synthesize it with things like market data and publicly available shipping information. Plus the CIA has (at least I hope it still has) a large staff of people whose only job is to study print, broadcast, and electronic media about other countries and compile that into regular reports of What Goes On There.

Obviously there's all sorts of covert information gathering that also goes on, but presumably the product of that is classified by default. Fortunately our executive branch is headed by intellectual types who enjoy reading and synthesizing a wealth of complex detail /s