nslookup has the -type flag to specify the type of information to lookup. -type=ns is used for NS (nameserver) lookups.
$ nslookup -type=ns <domain>
Example:
~ ยป nslookup -type=ns google.com
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
google.com nameserver = ns2.google.com.
google.com nameserver = ns3.google.com.
google.com nameserver = ns4.google.com.
google.com nameserver = ns1.google.com.
Authoritative answers can be found from: