nslookup -type=aaaa ipv6.google.com.
ipv6.google.com canonical name = ipv6.l.google.com
ipv6.l.google.com AAAA IPv6 address = 2001:4860:a005::68
Further, you can use PING6 or TRACERT6 for this.
Not many domain names would be associated with both, an IPv4 and IPv6 address. The trick is to find out related IPv6 domain names, most of them are under the .net and .org TLDs.
Here’s a huge list: http://ipv6.loercks.net/