Search their username in Instagram
Check whether their profile appears in search results
Open old DMs and see if their profile name changes to Instagram User
Try visiting their profile from an old message or tagged post
Look for missing posts, followers, or following counts
Check whether you can still mention or tag them
Try viewing their profile from a different account
Ask a mutual friend to check whether the account is visible
Compare results on the app and web version
Consider that they may have deactivated or changed their username instead of blocking you