If you want to know how much data is passing through your network interface, either because you have an Internet usage cap or you are simply curious, you need a network monitor. Darkstat is such a program and comes in two parts. the first part is the gathering of network statistics, which is handled by having the Darkstat daemon running in the background. If you install it through your distro's package manager it will probably have installed a service script to run Darkstat when you boot up, otherwise run "darkstat -i eth0" from your distros local startup file to have it collect information on network traffic.
The second part of the program is getting at the data and presenting it in a readable format. Now, Darkstat has no GUI of its own, which may seem a disadvantage, but fire up your browser and point it at http://localhost:667 to see clear graphs of your network traffic over the last minute, hour, day and month. Click on the hosts tab to see a breakdown of all the hosts that have connected to you, when and how much traffic they have sent and received, which can be useful if you suspect some form of foul play.
Using a web browser to display the data saves bloating the program with its on GUI code, but is has another advantage, you can access the data from elsewhere. Routers won't forward access on port 667 from the Internet by default, so you are safe in that only users on the local network can see this information, but you could set up your router and firewall to allow access from outside so you can keep an eye on machines remotely.