you can use the dig command in MacOS terminal with the @ argument to specify a nameserver to use for the query.
e.g. dig example.com @b.iana-servers.net SOA
will run the query against b.iana-servers.net.
You can compare the SOA serial numbers (ie 2013012703) between your current nameserver, and the one you want to add.
For example:
dig example.com @a.iana-servers.net SOA
dig example.com @b.iana-servers.net SOA
cwevans-home:~ cwevans$ dig example.com @a.iana-servers.net SOA
; <<>> DiG 9.7.6-P1 <<>> example.com @a.iana-servers.net SOA
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 41112
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0
;; WARNING: recursion requested but not available
;; QUESTION SECTION:
;example.com. IN SOA
;; ANSWER SECTION:
example.com. 3600 IN SOA sns.dns.icann.org. noc.dns.icann.org. 2013012703 7200 3600 1209600 3600
;; AUTHORITY SECTION:
example.com. 172800 IN NS b.iana-servers.net.
example.com. 172800 IN NS a.iana-servers.net.
;; Query time: 36 msec
;; SERVER: 199.43.132.53#53(199.43.132.53)
;; WHEN: Sun Mar 3 07:18:15 2013
;; MSG SIZE rcvd: 134
and then compare to the second nameserver:
cwevans-home:~ cwevans$ dig example.com @b.iana-servers.net SOA
; <<>> DiG 9.7.6-P1 <<>> example.com @b.iana-servers.net SOA
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17019
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 4
;; WARNING: recursion requested but not available
;; QUESTION SECTION:
;example.com. IN SOA
;; ANSWER SECTION:
example.com. 3600 IN SOA sns.dns.icann.org. noc.dns.icann.org. 2013012703 7200 3600 1209600 3600
;; AUTHORITY SECTION:
example.com. 172800 IN NS a.iana-servers.net.
example.com. 172800 IN NS b.iana-servers.net.
;; ADDITIONAL SECTION:
a.iana-servers.net. 1800 IN A 199.43.132.53
a.iana-servers.net. 1800 IN AAAA 2001:500:8c::53
b.iana-servers.net. 1800 IN A 199.43.133.53
b.iana-servers.net. 1800 IN AAAA 2001:500:8d::53
;; Query time: 39 msec
;; SERVER: 199.43.133.53#53(199.43.133.53)
;; WHEN: Sun Mar 3 07:18:22 2013
;; MSG SIZE rcvd: 222