Craigslist...
You can get a really good idea about what things are going for. When I was first starting I made a spreadsheet and tracked asking prices, I contacted a few and offered less to see if they would be willing to take it. After a month or two you will see the ads that are still listed asking too much. Compare the pictures and if they do not have pictures ask for them. In the DC area, we get flooded with interns looking for places just for the summer so a much higher demand but don't assume you will be able to get that price the entire year.