What are some warning signs that an area is slipping from C class to D class?

[Jeff G.]

I invest out of state in C-class neighborhoods in the Midwest. One of my units MAY be in an area that is going down hill, but I could absolutely be wrong. From several states away, how can I be sure? I'd hate for a temporary dip in the quality of the tenant pool to be giving my landlord "spidey sense" false alarms on account of taking an extra month or two to fill a unit. I'm happy to rent in a C-class area, even though it can take some work to stabilize a property. I don't really want to invest in a D-class area.

Tangentially, what are some of your "it's time to sell this property" red lines when it comes to the changing economic conditions of an area?