I know this is always a concern for authors, here and elsewhere...and believe me, I've been a victim many a time myself...
Lots of good resources on understanding, preventing, avoiding, and detecting plagiarism...
informED - Plagiarism
http://www.opencolleges.edu.au/informed/teacher-resources/plagiarism/
Rest in Peace
I once knew a drinker who had a moderating problem...
