If you are a buyer and have a "buyer's agent" or you are a seller and have a "listing agent" those realtors can then provide you with the most up to date and thorough information through the MLS (Multiple Listing Service.) In our area, this is where most transactions are occuring and are recorded promptly. Price is important, but through the use of the realtor you will also have information on square footage, updates, number of bedrooms and baths and any monetary credits that a seller may have provided to a buyer. This added information will be helpful in making a true comparison, if that is what you are intending to do.
Though the city of Janesville website and county records provide final selling prices, the information is not updated as quickly as within the MLS.