What are the indicators of a good/healthy rental market

[Dean D.]

Hi all, 

I am looking at finding my first "buy to let" property. What indicators can be used to guide in deciding where to purchase a rental property?

I can do a search on a property site for a given area, but what metrics can be used to determine if it's a good place to buy? 

Is it number of available rental units? 

Is it price of rental units? 

General quality of available units? 

Etc.

Any advice is much appreciated. 

Dean