It becomes obvious when you ask them to explain the code. At my university I once overheard a boy and a girl presenting some code "they" had written to a TA. The TA asked them some basic questions on while-loops and function calls. It became obvious that the boy had written all the code and the girl had no clue. So the TA decided that the boy had passed but that the girl had to come back and present the code herself on the next session.
