We used to page when tools.wmflabs.org was down, which was problematic because of the varying number of subsystems that could go wrong causing it to page. Instead, create a simple webservice that does the simplest thing possible (serve a page, probably?) and alert when *that* goes down. This could still be caused by:
- Proxy failure
- Webservice failure
- DNS failure
- Network failure
But is still a lot more robust than the main page check.