I mean, all functioning governments track the net value of imports / exports (that's the main job of customs departments).

Most functioning governments also keep a balance of payments (https://www.imf.org/external/np/sta/bop/bop.htm).

If the flow of imports / exports doesn't line up with the net flows of payments, someone would raise a flag.

So yeah, unless it's ML / illegal trades, we could at least figure out a way to track it.