How can i check which websites use a specific dns such as dns2.mysite.com? Thanks.

you need reverse IP lookup, try http://www.domaintools.com/reverse-ip/

you need reverse IP lookup, try http://www.domaintools.com/reverse-ip/
that will show hosted on which machine not using which nameserver

wiremind: its not possible unless you have access to each tld zonefiles and grep that nameserver against em