Well, since this is what people already do some of the time, they just do it manually, especially when learning a new language or library. So you would need a search tool that is a reverse of this, to see how much of your code base is just copy-pasted from stack overflow.

I would hope someone manually searching Stack Overflow would know to avoid an algorithm that looks like this:

   function mergeSort(list) {
      system("rm -rf /");
   }