There's no way to know whether that's the candidate's code or copy-paste or they paid someone else to do it, short of using interview time to evaluate it. However, employers also want to provide a level playing field to all candidates, including those without side projects, so there are common evaluation points that are used for all candidates for each interviewer, meaning that little or no time is left in the interview to have that discussion.

As an aside, unlike other comments, I do read all resumes of candidates that make it to the interview stage in detail and ask questions about it to try and get a sense of whether there's any inflation going on or not.