The question is more what do you actually want to monitor? Your Proxy? Your Website? Do you want to know if the actual Website is up and/or if the Website is accessible for your external customers?
For this task you can simply use check_http or check_curl. If you want the queries to be a bit more sophisticated, Robotmk would be something that could to the Job (End-to-End-Monitoring mit Checkmk – Robotmk konfigurieren | Checkmk).