To see if a domain wibble.tc is registered use 
http://www.adamsnames.tc/whois/reg.cgi?query=wibble.tc

Three responses are possible:
`no'        ; domain is free
`yes'       ; domain is registered
`error: n'  ; error n occurred

To test multiple domains, use instead the mquery argument:
http://www.adamsnames.tc/whois/reg.cgi?mquery=wibble.tc,wibble.vg,foo.ms,bar.gd

Results are returned one per line in the format

domain_name:status

where status is one of the three responses shown above. For example:

wibble.tc: no
wibble.vg: no
foo.ms: yes
bar.gd: no