How can I use dig(1) to test GeoDNS resolution for a specific IP?
There are google name servers all over the world and they use anycast routing so you will most likely get a name server physically close to where you are.
In addition google has introduced an extension to DNS called "Client Subnet in DNS Requests" https://datatracker.ietf.org/doc/html/draft-vandergaast-edns-client-subnet-02 .
I use http://check-host.net for this.
Eg:
http://check-host.net/check-report/2b21cc
It runs your query from many different geographic locations.