In most cases, it's difficult to know. Unless the platform or app specifically alerts the person whose story was screenshotted, there's no straightforward way to find out.
Well, usually it's hard to tell for sure. Some apps might have notifications or indicators, but not all do.
It's not that simple to determine. Different apps and platforms handle screenshots differently. Some might have built-in features to notify the owner of the story, but many don't have such functionality.