Do you have them upload the source code of their projects? If so, after the upload could you display the project’s file tree along with a last minute change their hypothetical client has requested and give them a minute or two to select the file(s) that would need a pull request in order to make the change? They don’t even need to make the change, just identify where the change would happen.

Someone who wrote the code themselves will know right away and someone who purchased a project won’t have enough time to sift through the code to figure out the answer.