Setup YourSites to check that your sites are up, that CSS and JS files and images are loaded and that key content appears on specific pages.
Be notified when a client site is down or that key pages are broken.
If the client site is behind a firewall then as long as the YourSites server is behind the same firewall or is whitelisted you can check your sites are up.
We love every single one of our users, without you YourSites simply couldn't happen! So we would love a review at the Joomla! JED so we can let others know about us too, please take a minute to write a review:
https://extensions.joomla.org/extension/yoursites-manager/
If you feel you have something negative to say, we would implore you to speak to us first, as we really really don't want anyone to be unhappy!