I guess you would hook 2 chips up as specified in the datasheet and test bit error rates at various bit rates and lengths of twisted pair cable. RS-485 is specified up to 1200m of cable and data rates up to 10 Mb/s although not as the same time. You may also need to test over the temperature range specified in the datasheet. You might get by with using one computer with 2 ports in a loop configuration.
