Is there a way to tell the difference between these random city levels in game?
As far as I understand, one cannot see the unit production of a city before deciding whether or not to pillage it.
So either you have played that map before and have good memory, or you are required to open the map editor and just look up the map's detail. I find the latter pretty tedious, but still important enough to actually do it for each of my games. It would be nice if one could just click on the city and get an indication whether the city is poor or rich, or what not.
Similarly, units with fixed production are known anyway, so why not show the initial production capabilities (not the current actual production capabilities)? One might argue that city X is famous for their Y units, hence it is common knowledge...