To check the bandwidth usage for your account, sign in to Virtualmin (see Account logins and important URLs). Form here you should see an overview of your Disk and Bandwidth usage on the main Virtualmin page.

For a more detailed breakdown of your bandwidth usage click Logs and Reports in the left-hand navigation, then click Bandwidth Graph.

From here you can view bandwidth usage by parent (meaning your main/master domain), Parent and sub-servers (your main/master domain and the domains thereunder), date, and month by clicking on the appropriate heading.