It's done on a simple scale from 1 to 5 with 5 being the fastest. If you go to your traffic page when downloading it will be listed under the speed column. It indicates the connection speed of the computer hosting the file.
I personally also look at the overall minutes to download and the bandwidth size to help determine which will be the faster files to d/l. Also make sure you're looking at the file size so you don't get an incomplete song, video, or movie.