Google has always been struggling to find out where content has originally been written. In his latest of his famous Webmastervideos Matt Cutts now explains some of the signals Google uses to figure that out. There is a part which you can help Google on yourself, for example by ‘pinging’ the search engines, but Google also looks at some ‘simpler’ elements. Like Pagerank: a higher Pagerank often indicates the source lies there.
Then off course there is rel-canonical which is a huge indicator, but other elements like rel=author also play a big role. Cutts ends with explaining that there are a lot of signals, but that ‘even’ Google messes up sometimes.
Take a look at the entire video Cutts, who was promoted within Google to “Distinguished Engineer” at Google, whatever that may mean, made: