To determine whether a domain is already registered you must perform a "whois" lookup.  If the lookup returns no registration then the domain is available. If not the tool will return information about the registrant. Just because there is no website for a domain does not mean the domain is not registered. 

This is known to work with .com, .net, .us, .info, .cc, .me and it will probably work with some others. 

Some domains take longer to return information so please be patient. 

Remember: enter domains only here - no www or other hostnames!