Is a Walmart or Home Depot in the area ever a consideration? These guys do their research and if they're willing to build in an area, it must carry some weight.
For example, I'd be more comfortable buying a property in a 10k population town (outskirts of bigger town) if it has a Walmart.
I would start by looking at the macro market's fundamentals, that is the population, job and income growth over time.
After that, I would look to see what the rents and home prices are and how those are trending as well.
I would do a similar analysis, but at the sub-market level for the metro area you are considering flipping in.
You should also look at comps via sites like Redfin or Zillow to see what others are doing in the space, how much they are paying for their flips, and what the ARVs are looking like.