i-rule question

In my opinion, the easiest way to check this, whether the resource you want to check is the destination address or not, is to create a pool with a pool member at that IP address. Then, configure an advanced health check monitor to mark that pool member up if it returns "OK" and down if it returns anything else. You can check this pool member in an iRule if you need to and then do whatever you need to based on the status of the pool member.

If you need assistance with the configuration or any more questions, I am sure I can help.