There's several ways to go about this. Zillow and Trulia both publish rental data for a specific property which are based on rents from the surrounding area. If you can trust those, then you're basically letting those sites do the homework for you.I usually compare them with specific rental listings (which also appear on both sites) to make sure the expected rent is in line with what's being asked for currently.Both the area data and the specific rental listings paint a picture for me as to how much something should rent for.