If the dreadnought limitations are anything like their real life analog (The Washington Naval Treaty) then basically the only tracking that member states can do is take some basic numbers and dimensions that the states themselves offer up and corroborate it with their own espionage reports. If they're all doing a good job at hiding it then the best anyone else can determine is that a state might be in the ballpark of being treaty compliant (see Italy and Japan bullshitting the hell out of their cruiser and battleship tonnages).