The Wiktionary API can be used to query whether or not a word exists.
Examples for existing and non-existing pages:
http://en.wiktionary.org/w/api.php?action=query&titles=test
http://en.wiktionary.org/w/api.php?action=query&titles=testx
The first link provides examples on other types of formats that might be easier to parse.
To retrieve the word’s data in a small XHTML format (should more than existence be required), request the printable version of the page:
http://en.wiktionary.org/w/index.php?title=test&printable=yes
http://en.wiktionary.org/w/index.php?title=testx&printable=yes
These can then be parsed with any standard XML parser.