There will be an additional icon on the system tray whilst avast is scanning your email, see image.
The avast ' a ' icon will also rotate whilst scanning.
You can also open the on-access protection control (right click the avast 'a' icon, select On-Access Protection Control), click the Details... >> button and select the Internet Mail provider and you will see the changing details. You can also hover the mouse over that other icon I mentioned and it will show tooltips.
The same drill for the access to the Web Shield settings Pause/Terminate or right click the avast 'a' icon, select Pause Provider or Stop Provider and select which one.