Search their username and see if the profile appears
Check whether their profile page says “No Posts Yet” or seems unavailable
Try viewing their posts from a mutual account
See whether their comments or likes on your posts disappear
Try sending them a direct message
Check whether old chat threads still show their profile name and photo
Look for an error message when visiting their profile
See whether you can tag or mention them
Try following them again if the follow button appears
Check whether their account is visible from another Instagram account