Detecting upstream server failures

[John Moore]

I'm using the nginx as a reverse proxy server, with the upstream module, 
successfully doing round-robin load balancing between two backend 
servers. Is there some way I can determine if one of the back end 
servers has gone down? Nginx will seamlessly take it out of pool if it 
does go down, which is great, but it would be handy to know about it so 
some alert could be sent to the admin.