Search their username in Instagram
Check whether their profile appears in search results
Open a past DM conversation with them
See if their profile photo, posts, or follower counts are missing
Try visiting their profile from an old link or tagged post
Check whether their posts and comments are no longer visible to you
Try following them again
Ask a mutual friend to check if their profile is visible to them
Look for messages that still show but no longer let you view the profile
Consider that they may have deactivated or changed their username instead of blocking you