Use the canonical tool to check if a page has a canonical tag and to find out which page should be index by search engines based on the canonical location.
The syntax used for a canonical tag:
The canonical tag is supported by:Google, Yahoo, and Bing.
UPDATE: This tool now also supports / checks the HTTP Canonical headers
Read the official blogpost by Google on Google Webmaster Central: http://googlewebmastercentral.blogspot.nl/2009/02/specify-your-canonical.html or watch the video explanation from Matt Cutts: