Networks

Or see the man page or other docs to use it as needed.
http://www.google.com/search?ie=UTF-8&oe=utf-8&q=linux+dig

Here's a web man page:
http://linux.die.net/man/1/dig

I assume also that these servers have IP addresses. The IP address from which the response is sent tells you which server it is.

You can also do packet capture and analysis on your network to see what is going where. Use Wireshark or something like that.

Bottom line: I have no idea what you are trying to do, under what circumstances you expect to be able to identify which server is responding, etc. Also, I am neither a Linux nor BIND admin.

But, in short, you should know which is responding because you have to configure the nameserver address in the OS or router to resolve names anyway. Primary and secondary nameservers are authoritative for the zone. They respond to recursive (caching) nameservers (resolvers), and resolvers respond to lookups from the network.

So, in short, it is always the caching server which answers the lookup, unless it is the resolver doing the lookup against the authoritative NS, or against another resolver (e.g., forwarded to an internet resolver).