There’s no official notification, but a few signs usually make it obvious if you search their username and it doesn’t show up (but it used to), or you can see their profile, but it says “No posts yet,” and you can’t follow them, that’s a strong hint. Also, old DMs may still be there, but their profile picture might disappear, and tapping their name won’t open their account. I’ve noticed the easiest way is to try viewing their profile from another account. If it shows up there but not on yours, you’re most likely blocked.