I would also consider a practical approach. For example if I wanted to know how quickly my LAN could move music files and folders around. I would take a suitable sample collection of music files and folders, measure the size of the collection (using ‘properties’) and simply time how long the transfer takes.
both Windows and OSX have built in graphical monitors for the network connections. For windows it is part of Task Manager, for OSX it is called Activity Monitor (I think, don’t have Mac to hand at work).
For my own Gbit network, I rarely see anything sustained above 500Mb/s. I have a lot of legacy Cat5 cable in the network though, so that is fine under the circumstances.