There are various websites that allow you to check whether the nameservers have been configured correctly. The best way is to check this with the register. Below you'll find a number of frequently used extensions.
.nl - http://www.sidn.nl/nc/over-nl/whois/
.be - http://www.dns.be
.eu - http://www.eurid.eu/en/whois-search
.com, .net, .biz, .org, .info - http://whois.icann.org/
.uk, .co.uk, .me.uk, .org.uk - http://www.nominet.org.uk/uk-domain-names/about-domain-names/domain-lookup-whois/whois-tool
Using the websites below, you can perform a DNS check to see whether the nameservers are configured correctly. You can also use this website to lookup other DNS information.