For completeness - that's just one grid included in this viz, for the map of the different european grids see: https://en.wikipedia.org/wiki/Synchronous_grid_of_Continenta...

(Russia is another big chunk of Europe but not included in the above map, though I think it may be the same grid with the Baltics)

For the question, measuring the flow of electricity is just normal power engineering, your home electricity meter does it as well. It's not so much the countries doing it though.

The Baltics are disconnecting from the Soviet era BRELL grid with Russia and Belarus in two months [0].

https://www.reuters.com/world/europe/baltic-countries-leave-...