well i can think of only these obvious things such software would detect:
1. formatting of text.. if we copy text from different sources mostly the source formatting is also copied. so if a document mas multiple such formatting where it should not be then we can say to an extent that it was copy pasted.(this can be easily be avoided by copying the whole text on to a notepad/other software which dont have formatting capability and then copied back to ur fav editior and saved.)
2. Grammer/writing style.. well everyone has that speical way of writing stuff just like biometrics. so if in a document content(style/grammer/special words) is not uniform or consistent then it can be classified as plagiarism. (this might be avoided by writihg the whole document again in your own words after u have assembled if from various sources, may be this wont technically be plagiarism and hence undetectable)
3. Google... well only if google has located that exact document/portion/phrase somewere then we can go there and compare the document at hand and data located by google. (again rewrite to avoide this)
what else could there be....
i think the best test for plagiarism would be a final review/inerview that will tell the employeer/examiner whether the person is capable of writing what he wrote.
just using logic.. i dont know anything about plagarism or such software.what do u think.