domain - Verify that a name server is registered at a TLD registry

Let's say I own the domain name example.com. It is registered at registrar R. In my account at R, I registered the following name servers for the .TLD registry:

• ns1.example.com --> 192.0.2.1


• ns2.example.com --> 192.0.2.2

Let's say there are no domains using these name servers for DNS yet. How can verify outside of my R account, that the name servers have been registered successfully at the .TLD registry?

For example, VeriSign Inc's WHOIS page allows you to look up a registered .COM name server:

Server Name: NS1.HOSTGATOR.COM
IP Address: 67.18.54.2
Registrar: ENOM, INC.
Whois Server: whois.enom.com
Referral URL: http://www.enom.com

Is there a standard approach for looking up this information for any TLD registry?

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To clarify, I am not looking to verify the name servers a domain is set to use, e.g.:

Domain Name: HOSTGATOR.COM
Registrar: ENOM, INC.
Whois Server: whois.enom.com

Referral URL: http://www.enom.com

Name Server: NS1.P13.DYNECT.NET
Name Server: NS2.P13.DYNECT.NET
Name Server: NS3.P13.DYNECT.NET
Name Server: NS4.P13.DYNECT.NET

Status: clientTransferProhibited
Updated Date: 05-jan-2013
Creation Date: 22-oct-2002

Expiration Date: 22-oct-2015

Instead, I am looking for a way to query a single registered name server (e.g. ns1.example.tld) against the registry, to find the IP address the registry has recorded for this name server (e.g. 123.456.789.001).

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I have contacted VeriSign for input, and it turns out that VeriSign's WHOIS lookup for name servers is proprietary. Here is their response:

In regard to your question whether you would be able to obtain any
information regarding a nameserver which is registered at the registry
but not currently associated with any domain, you will unfortunately
not be able to obtain this information via alternative routes e.g. by
performing a DiG. Nameservers are only published to the zone if they
are attached to a domain, hence you won't be able to obtain this
information unless said nameserver is associated with a domain. Also,
a DiG may provide you with a listing of nameservers associated with a
domain and their respective IP's, etc.

-- Benjamin, VeriSign, Inc. Customer Service

The second part of @Iian's answer below is correct. The name server must be associated with a domain in order to lookup the IP address the registry has on record for that name server.

Answer

You can us dig and choose a public DNS server

dig 8.8.8.8 NS example.com

This will get you the names of the name servers then you can just look them up e.g.

dig +short ns1.example.tld 
123.456.789.001

or if you're using windows you can use nslookup

nslookup

> server 8.8.8.8
> set type=ns
> example.com 

Then similar to above

nslookup ns1.example.tld

will provide you with the ip address

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If you want to query the tld directly then

dig ns tld

will provide a list of the TLDs name servers, you can then query them directly for the ns records for example.tld

dig @ns666.tld ns example.tld