Once I read a corollary of the idea "follow the money." You can tell a city is going bad when the wealthy people leave. If a bunch of them do it, what remains is on the way down. A little bit because rich people fund a city, but mostly because many people want to leave but only rich people can leave quickly. Again: not because rich people are smarter or better, but because many really smart and really great people would like to leave but can't afford it.

Watch for when the rich people leave.