Best tools to evaluate the earning potential of a rental?

[Bethany Lewis]

Does anyone on Bigger Pockets know the best tools for a Real Estate Investor to assess the earning potential of a rental property to determine if it's a smart purchase, and whether or not it should be used for long or short term rental? I've heard a thing or two about Mashvisor.com but not sure if it's the best tool out there. 

Thanks!