Many programs refuse to go beyond a trial period. Even if they are trashed and then reinstalled, they 'remember' that the trial period has expired. Assuming no contact with a licensing server, what is the general way that most copy protection works? Do programs drop files in random folders on the hard disk that are hard to track down? I know there's no registry on OSX/Linux, but perhaps something similar... ? Or must it be a file/folder?

I'm actually not curious from a hacking side but rather from the implementation side, but in any case the question is basically the same.