avast should scan all files before they are opened. You should see the avast icon rotate when files are scanned. You should also see the ‘Scanned count:’ total number of files scanned increase if you have the Resident Scanners window open.

From this image you should be able to see it and if the Standard Shield is active.

http://img.photobucket.com/albums/v325/for-dwr/avast-scanners.jpg