Drupal Sentry is a centralized site-monitoring tool, to help watching over the health of all your sites. Install this (the server component) on the central server, and use the client on the sites you want to monitor.
After installation you can create 'sites' that hold essential information (update periodicity, site url etc). Be mindful that updates are triggered by cron, therefore they can only occur as frequently as the cron of your sentry server site (eg.: you can't poll your sites every half an hour if the cron for the server is set to once per day).