Standard tools will allow you to ping the IP or probe the HTTP port - these are cheap & simple ways of verifying that the web service is minimally available. In order to validate that they are also fully functional, you will need to do a bit more... your monitoring package will need valid credentials to login to the various web services, not to mention specific proxies & business logic to execute against each one.
Have you looked at MOM (especially MOM management packs)?
