Search for their profile on Facebook
Check whether their profile no longer appears in search results
Open an old chat thread and see if their name or profile picture is missing
Try sending them a message and see if it fails to deliver
Look for their name in mutual friends lists
Check whether their posts, comments, or tags disappear from your view
Try visiting their profile link directly
See whether you can still tag them in a post or comment
Check whether they appear in your friends list
Ask a mutual friend to search for their profile from their account