The best way to do healthy check in forward proxy?

[Igor]

Hi, all.

I got 2 squid forward proxies, and just want to use haproxy to do load
balance and healthy monitoring.
I got basic backend conf like:

  balance roundrobin
        server  squid1  127.0.0.1:999
        server squid2  127.0.0.1:998

but what's the best and smart way to config haproxy to do healthy
check to make the high availability of the proxy server?

Bests,
-Igor