Monitoring hosts behind a load balancer

Greetings, I'm monitoring several Solaris 10 servers which are behind an F5 load balancer. One of the features of the F5 is that it answers pings for all of the hosts behind it, even if they are all down. Has anyone devised a method of testing the servers for being alive in this context? There are no other network based tests being done. Yes, all of the other tests eventually go purple in Xymon, but we'd like to know that the hosts are down when that happens.

Thanks, Steve Holmes ITaP/Purdue University