Search for their profile on Facebook
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Open an old chat thread with them
See if their profile picture and name still appear in Messenger
Try sending them a message
Look for a message delivery status change
Check whether you can tag them in a post or comment
Try visiting their profile link from an old comment or message
See if their profile page says content is unavailable
Ask a mutual friend to search for their profile
Check whether their name still appears in mutual groups or old posts
Try adding them as a friend again
Check whether their reactions or comments on your posts are still visible
Compare results from another Facebook account if available