The title of this thread may seem like an easy question, but unfortunately for many webmasters, Google's algorithm in many cases doesn't seem to be able to distinguish between original and stolen content within its index. With so many sites "borrowing" content these days in order to generate revenue (see Barry's last post about MFA's for more info on this), the problem of website owners finding their content reused and indexed above them in the rankings is growing exponentially.
A thread at Search Engine Roundtable Forums that started last week gives an example of one member's frustration, as they say:
It is pretty lame if someone (who) just copied content gets better SERP than the one who made the original content..
Member “Sprouty” envisions an army of humans trying to solve the problem at the Googleplex. It would seem to be a simple issue of assigning priority for particular content to the first site which it was indexed, but therein lies the problem. Sometimes the plagiarism happens so fast that the first site crawled actually copied the content. This poses yet another challenge for people adding content, especially as a part of their search engine optimization efforts. It would seem that you would want to wait to release new content until just before a scheduled crawl, or use the Sitemaps system to submit the content directly to Google.
Hopefully there will be some more good thoughts and advice on this topic as the discussion grows at SER Forums.