Picking a "out of state" market for rentals (multi-family)

[Andrew Lawrence]

I know there's a lot of posts already dating back asking about which markets are good for someone from X state to invest in Y or Z states for good cashflow etc etc. But I am wondering the why behind it. Some posts mention fundamentals of a market or growth vs declining markets. 

How does one look at a market and decide that it fits the 1% (or more) rule to determine which market has some of the highest ROI?

What are some good sites for determining value like this? Population, unemployment rates, potential big companies moving in = more jobs, Rent rates, Etc?