Identifying legitimate emails from the iTunes Store - http://support.apple.com/HT201679 - lacking in specifics, the next link is better:
Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - https://support.apple.com/HT204759
Forward attempts as an attachment to: email@example.com
- Apple e-mails address you by your real name, not something like "Dear Customer" , "Dear Client" or an e-mail address.
- Apple e-mails originate from @apple.com or @itunes.com but I think it is possible to spoof a sender address
- Mouse over links to see if they direct to real Apple web sites. Do not click on them as this just tells the spammer they have a working e-mail address in their database.
- Phishing emails may include account suspension or similar threats in order to panic you into clicking on a link without thinking. They may report a fake purchase in order to infuriate you into rashly clicking on a false link to report a problem. Mar 2018 post by Niel https://discussions.apple.com/message/33129140#message33129140 - "Emails saying that your Apple ID has been locked or disabled are always phishing. If one actually gets disabled, its owner will be told when they try logging into it instead of through email."
- Apple will not ask for personal information in an e-mail and never for your social security number anywhere.