you can use copyscape.com to search for duplicate content but I ussually use Google - grab
a sentence or two, put them in quotes and do a search on Google - if you get many results that means that the content is not original, for example:
http://www.google.com/search?q=%22Se...nch%20black%22
gives me 16 or so results which menas that the same text is used on many sites
Clever dudes believe that changing 2-4 words in a sentence with synonymous words will fool Google and make your content unique, same example (changed 3-4 words):
http://www.google.com/search?q=%22Se...nch%20ebony%22
you get 'No results found for....' which means you can use this piece of text on your blog and hope that the clever dudes are right
ama Mitko, tova go znae vsiako hlape ;)