Yahoo site Explorer is a tool by which we can find the source of competitors list.Record the source link and save them for later.

Alexa.com also would give you information on this. seomoz.com looks good.

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I think alexa is a good resource also.

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webconfs.com site help may help you to know the source of your competitor's link

I think The best tool to find the source of competitor's backlink is opensiteexplorer.org

I agree with opensiteexplorer.org you can also compare your website's authority with your competitors, the only problem is.. limited to 1 search per day I think if you are not a Pro-member.

The best tool is the paid one. The free version will not give you complete information.

i use this tool
www.backlinkwatch.com

Opens site explorer and backlink watch are the great tools for checking the competatior analysis.