One of the easiest way to check the bandwidth or data transfer on any network is by checking the data transfer in your routers admin panel. However for any reason if you’re not able to do that, or if you want to be able to access the data transfer details on your mac os x directly from your macbook, you can do that by a built in app for data transfer.
Activity Monitor – mac built in app
You might have used this app before too for checking system load and other stuff.
This app also has a tab for Network which shows network and bandwidth details.
This tool tells you the details of packet in and packets out plus data received and data sent along with a graphical representation.
However, there is one limitation that this tool shows you the data transfer of current logged in session too. After logout / restart of your mac os x it will get reset.