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.
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.
Maybe. It depends on if they have any way of tracking or knowing that you rewatched it. Some snaps might have view counts or indicators, but not all do.