If you look at city details it says devastation, that is the amount they reduced the income. It only shows up if there was devastation done either by an enemy army or rebels.

As far as the amount of damage they can do. If it's farmland they take a big chunk out of your income. If it's a road it takes a decent chunk out. If it's the wilderness they are nor really worth going out of the way for as the reduction of income is usually less than 50. Once rebels have done the max amount of devastation they look for a new area to move onto wich always results in them moving either to a road or farmland.