You may try examining the HTTP headers of responses from the server to see if there are any clues. Standard proxies (like
squid) can be configured to insert a
Via: line or similar in the headers. Most likely a reverse proxy isn't going to do this or is going to be configured not to do this, but it is worth a shot.
Pinging a web server acting as a reverse proxy will reveal the IP of the "front end" server and NOT the server behind the reverse proxy. ICMP does not travel over or have anything to do with HTTP so there is no way this could happen.