Play tenant. Drive the area you are wanting to invest in and start calling the places that are for rent. Then see how long it takes for the place to rent. If it takes a real long time, they may be asking too much.
Good advice above. Also, it may be worth while to ask slightly below market rent to get a good number of qualified applicants. Screen well and get a superior tenant. They are worth more than a $25-50 premium you may be giving up.
BP has already assembled a list of websites you can use. Go to the menu on the right, and select "Real Estate Tools", then go to the "Fair Market Rents" link!
I only see tenant screening, mortgage rates, task manager, and find foreclosures
Those are the links under "Tools" at the top of the page, near the banner area. I am describing the links going down the page at the right of the posts. Just below a heading called "BiggerPockets Resources", you should see what I described previously.
EDIT: Shanequa has just posted an alternate way of getting to the same page; the "Resources" menu she mentioned is the one at the top of the page.