Neighborhoods are often classified A,B, C and D. Investors often like to look at their investments through rose colored glasses and a D sometimes becomes a C.
There is no official rule, but here is how I would look at this: find out what the median price is for a city. You can look on Zillow or Redfin. The median price is the dividing line between B and C. Anything below median is C or D, anything above is A and B.
Look at the distribution between C and D, determine the median (you can ballpark it or use excel), that's the dividing line between C and D.
So for Milwaukee County the Median Sales Price is currently 219,000 which get's you approximately the following for single family homes:
class D: 0 to 140,000
class C: 140,000 to 219,000
class B: 219,000 to 300,000
class A: 300,000 to 1,500,000+
If you include all of the Milwaukee Metro area (which is bigger, about 1.6 million population) the median increases from 219,000 to 265,000. Milwaukee proper would be a bit lower. Of course that moves the whole classification system with it, so you can see it's not an exact science. I tend to prefer larger datasets though, as smaller sets are often more theoretical.
