If the program is non-trivial (e.g. contains some classes, many functions), then usually the program structure alone will give it away (1), unless you invest a

If the program is non-trivial (e.g. contains some classes, many functions), then usually the program structure alone will give it away (1), unless you invest a lot of time into changing things. From my experience (as a student), the people that copy programs don't even work that hard to obfuscate their plagiarism. Usually they just rename a couple of variables and reformat things a bit. A former professor of mine wrote a program to "fuzzily" compare two source codes and show how close they are, which quite nicely showed of clusters of programs where a bunch of students had copied-and-slightly-modified from one student. (1) i.e., give the instructor a feeling of "Hm, I've seen that before...".