It depends how much you want to get into it and how big the project is. If it's finding only identical phrases or text, most word processors support this, and finally a lot of programming ide's support regular-expression searching (http://en.wikipedia.org/wiki/Regular_expression). 
On the other hand if you are looking to find reworded or similar paragraphs and/or excerpts and want to automate this (rather than substringing and searching for variations manually), than I would recommend you look at http://www.nltk.org/

`NLTK` is a toolkit that integrates a whole range of language manipulation, sorting and tagging tools - I realize it may be more in depth than you want to go (but it is quite easy to learn and has a very good beginner friendly documentation http://www.nltk.org/book/). 

I'll let others comment on more generic and perhaps quicker to handle at a glance tools, as I have no experience with those.