Most of the time, people can't tell. However, if you have a pattern of always rewatching and maybe responding quickly or frequently, they might have an inkling.
Generally, people won't be able to tell if you rewatch their Snap story. There's no built-in notification system for that. However, it's always possible that in the future, Snap could introduce such a feature.
As of now, Snapchat doesn't have a feature that lets someone know if you've rewatched their story. It's designed this way to give users some privacy when viewing content. For example, if you accidentally skipped a part of a story and want to go back to it, you can do so without the creator being aware that you're rewatching.
It depends. Some Snapchat features or third-party apps might give clues, but it's not always straightforward. Usually, there's no direct notification for rewatches.