Search their username in Instagram
Check whether their profile appears in search results
Open any old DM chat with them
See if their profile picture, posts, or follower count are visible
Try visiting their profile from a browser while logged out
Check whether their account says “User not found” or “No posts yet” unexpectedly
Look for missing comments or likes from their account on your posts
Search your followers and following lists for their name
Ask a mutual friend to check their profile
Note that Instagram does not provide a direct blocked-by list