It will all depend on the methods used by the individual registrars.
The definitive source should be the 'whois' service, so always check that first. This will list the names of the nameservers currently listed for your domain.
After that, check that the TLD's name servers have been updated by the registrar. On Unix:
% dig @<tld-server> <domain> NS
.com might be
a.gtld-servers.net, and for a
.uk domain would be
If you don't know what your TLDs name servers are, use this:
% dig @a.root-servers.net. <tld> NS
At this point, you should have been able to confirm that the higher levels of the DNS hierarchy have the right information - namely the current name servers for your domain.
For each such name server, then run:
% dig @<nameserver> <domain> SOA
% dig @<nameserver> <domain> NS
The first command will return the
SOA record. In general each of them should have the same serial number present. If they don't, then their servers are not yet synchronised.
The second command returns the authoritative version of the
NS records as maintained by your domain registrar. In general these should match whatever's in the TLD's DNS servers.