de-delegateing name servers

How do I check that a customer has removed their name server delegations for the name servers under their domain name? There is a site where you can search this but I forget.

Example, customer has name servers set up

ns1.theirdomain.com <myip>
ns2.theirdomain.com <mip2>

I want to make sure the ip numbers have been freed up so I can use them for other name servers.

Thanks

 

 

 

 

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