Yes , its optional . 1-http://www.copyscape.com enter the site which you are susspicous. 2-http://www.goole.com "" in brackets type each sentence thats another option. This is what i do , on couple of my sites.
1. Hire a real writing professional (preferably in a country with solid IP laws like the USA, UK, etc). 2. Get a contract. Copyscape and google are not that great. Better than nothing? Sure, but they don't even begin to do the job. They "might" find the most blatant attempts at theft. However, anyone with half a brain can steal an article and change it so these services won't show it.
If it's changed enough, it's obviously legal. Every writer gets information from other sources and write articles merging and editing info that came from others. The problem is how much should be changed, and that's the problem. If the above methods are used, I think that it's enough.