One way to tell is if you can't view the stories of someone you used to be able to. Another indicator could be if your comments or likes on their posts don't show up. Sometimes, you might not even be able to follow or message them if you're blocked from their stories.
You might notice that you can't see the person's stories when you usually could. Also, if you search for their profile and don't find the stories option, it could be a sign.
If you're blocked from Instagram Stories, you'll likely find that when you go to the person's profile, the stories section is missing or unavailable. Also, if your attempts to interact with their content, like sending direct messages or tagging them, don't go through, it might suggest you've been blocked.