I guess your question is : how do I know many rentals in one are

I guess your question is : how do I know many rentals in one area, few in another?

I pay attention to my local area.

1) Drive around, find many "for rent" sign in one zip code, and few in others.
2) craiglist
3) MLS list from my realtor.
4) local government appraisal web site. If the owner address is different from property address, it is good chance that the house is a rental property.