You could use something like Curl or Wget in a script to periodically poll the web location and parse the output to determine if it is what you expect. Those results can be fed into an analysis or e-mail alert.
I have scripts like that which I used for network ACL and firewall port validation. Say we get a new app for our users and the network team sets access to it. I can run scripts from the POV of the end users to validate whether or not they can actually access the intended resource. This has been great from an audit perspective. (This has saved my bacon multiple times as I could demonstrate the failure of the new app was due to ACLs or firewall, not the app or the install process.)
For non-web apps a tool like PortQry or even Powershell can be used to tickle specific ports.