You might figure out someone blocked you on story when you notice that their profile doesn't show up when you search for it, or if comments or likes you left on their previous stories disappear. But keep in mind, it could also be due to other technical glitches sometimes.
Well, one way is if you suddenly stop seeing updates from them in your feed when you used to. Another clue could be if you try to access their profile and the story section is unavailable or shows no content for you specifically. Sometimes, mutual friends mention something from their story that you have no idea about.
Well, if you used to be able to view their story and now you can't, that could be an indication. Also, if you try to message them and it doesn't go through or shows as undelivered, it might mean you're blocked from their story and communication.