How to check backend servers thru Haproxy?

Hello,
I have following configuration installed:   


                                                                     +--   backend 1

Internet  ---- Reverse Proxy with Haproxy      |
                                                                       +--  backend 2



For each backend, I have a specific cookie (SERVERID) set. These backends are monitored by loadling a picture (gif file) using HTTP protocol. Sometime, for maintenance reasons, I rename the gif file (to bring the backend down in Haproxy). Then, I perform some maintenance tasks. Before bringing backend UP (set gif file name to its original value), I would like to know if it is possible to check health of this specific backend thru reverse proxy. This is to avoid issues for end users who are on Internet.

Thank you for any tips or response that might be helpfull.


Jean Michel.