By querying the authoritative DNS server.
Ehm.... Aha
)))
Of course, my stupid question. Thank...
Not a stupid question at all; most people are not smart enough
to even ask it.
[It was a 'naive question' (which is not a criticism and) which
means that it came from a lack of experience or sophistication
on the subject. Keep asking....]
Btw, do you understand that the first time you query a non-authoritative
server (e.g., your own DNS server when looking for an external
zone domain name) then it MIGHT give you an authoritative answer
if it had to go ask the authoritative server.
That is, if the name isn't already in cache on your own server.
The next time (in a short period) it will practically always give
back the cached or non-authoritative answer (unless TTL expires
or someone explicitly clears your server cache, etc.)