I did find it, although calling it an evidence is too much, a mathematical guess is the right definition. This is how: mouse over (hanging it) the production value of a city in the city "screen". If you see a factory bonus that is a multiple of 3 more of what the city is supposed to have, then you have the bonus from the factory in another city's district. For example if you have +3 while not having any factory in the current city, or +6 , etc. Same applies to the power plant.