How to Estimate Rent for an Investment Property?

[Iman Yu]

Here is a good article I found on how to calculate cash flow when you purchase a investment property:

http://realestate.about.com/od/knowthemath/qt/rent...

it's very useful and I want to share it with every one.

My questions are: 

1) How do you know how much rent you can get from the property you are going to buy?

2) How do you find out  other properties that're similar to the one you want to buy are rented out for?

Thanks for helping out a newbie like me! :-)