It’s turned on by default.
You can left click the ‘a’ blue icon, click Details button and see WebShield at left.
At right you’ll see the last file/url scanned by WebShield.
But manually monitor the site? What do you mean, what do you need?
I remember, I am pretty sure, that Avast was used to scan several pages of my web site this past year and I saw the scanning happening realtime.
I originally came across AVAST because my site was reported as an attack site with potential malware and AVAST was recommended to me as a way to find what page the problem was on.
It worked beautifully, and now I want to do that again, as it’s much easier than scanning my entire home directory, since I know from the google report that their scan took place recently.