well it depends on wich operating system the endpoint are running. if thy are all windows/windows server you can use
http://download.cnet.com/LAN-Speed-Test/3000-2085_4-10908738.html it's freeware, it simply copy a file froma A to B and measure speed.
while testing the network segment must be as more as possible inactive,(eg. no backup running.)
try and let me know if i got the point
paolo