Plagiarism checks for source code
I have been responsible for using performing and evaluating tools for detecting software plagiarism in my academic department.

There are several published review articles in the scholarly journals. One such report Culwin, MacLeod &amp; Lancaster, 2001, UK JISC, "Source Code Plagiarism in UK HE
Computing Schools,
Issues, Attitudes and Tools" gives an overview. 

Basically, there are published algorithms and methodologies you could implement yourself, open source software you can run locally, or an internet based service you can register and use.

One such tool that you can run locally is JPLAG from the University of Karlsruhe in Germany. This has several language syntax implemented, and as it is open source you can amend as desired.

The best online tool is MOSS from Stanford which is the one I ended up using. 

If you wanted to research the various published techniques for detection, Google Scholar is a good source.