One sign is that they have no profile picture or very generic pictures. Another sign could be that they follow a large number of accounts randomly but have very few followers themselves.
Well, on Instagram, there are often a lot of accounts out there. 'Random accounts watching ig story' might refer to those accounts that you don't really know or have any connection with, but they happen to come across and watch your story. It could be because of Instagram's algorithm that shows your story to a wider range of people, including those random ones.
No. When an Instagram account is restricted, its access to various features, including viewing stories, is limited. Instagram restricts accounts usually due to violations of its terms of use or community guidelines. So, if an account is restricted, it won't be able to see your story in normal circumstances.