How do I detect if a pool is down in an iRule?

The way to do this is to write a handler for the LB_FAILED event.

You can then query the pool and take action according to which pool was selected.

Eg:

 
 when LB_FAILED { 
    switch [LB::server pool] { 
       pool-customer1 { HTTP::redirect http:://foo/customer1 } 
       pool-customer2 { HTTP::redirect http:://bar/notavail } 
       default { HTTP::redirect http:://sitedown/ } 
    } 
 } 
 

You can also use "HTTP::respond" instead of "HTTP::redirect" and then you can completely customize the response.