Go to Windows Start Menu and find the browser icon. Right click on the icon.
If that browser set to be sandboxed, then there will be these two choices.
Run outside sandbox
Always run outside sandbox
If it isn't, then these two choices are presented in the context menu.
Run in sandbox
Always run in sandbox
Whether or not a program is running at the time, a program set to always run sandboxed should appear in the AvastUI on Security Tab | Tools | Sandbox when you click on Settings there. Click on Virtualized processes and they should appear there. Note that if that settings page is open at the time you set the program to run virtualized, the page is not updated until you go to another part of the AvastUI and then return.
There does seem to be a quirk with Opera 32 and Opera 64 bit both installed. I set each to run always sandboxed via the Right click context menu. Sandboxing each required the selection of each version's icon as expected. However, using the Right click to Always run outside sandbox on Opera 32 bit resulted in Opera 64 also not being sandboxed. That's not a killer flaw unless it's not noticed.