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Unless your unfortunate and live in Florida and you have to bring into the Apple Store if you did not get it from Apple.com.
 
Visit this url to check your AppleCare status. You can then get a text message or email sent to you with a link to open on your iPhone to run the built in diagnostic app. After it’s approved, you can then buy AppleCare+ online. I just did this with my new iPhone X.

https://checkcoverage.apple.com/
 
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