You can ask algorithmic questions that are not easily googleable or standard problems (basically where they have to invent the solution themselves). Although it takes some effort to find the right difficulty problems (tip: the right difficulty is pretty low) so you don't end up wasting time on stuff nobody can solve or stuff everyone easily solves. You can ask people you know to take a stab at a problem to gauge its difficulty. Or you may even be able to eyeball it.