Having a valid SSL Certificate for your site is important, but you still need to to make sure that users, and search engines, see the secure version - and only the secure version.
Any attempt to access your site using a http address should receive a 301 ("Moved Permanently") status code and be automatically redirected to the equivalent https address.
Every 30 minutes, SiteSentry will scan your site to check that an attempt to access it using a http address is redirected to a https equivalent. If it doesn't get redirected, SiteSentry will send you an alert.
This check is a great complement to the SSL Certificate Expiration check, which ensures that your site has a valid SSL Certificate in place.