How can I know if a mail from Apple iCloud is legit or not?

Here's a tip I learned - Tap on the From field to reveal the real From address. In this case, my "Apple warning" looks like it came from Apple Customer Service. But tapping on that, I see it is really from the crazy address you see at the bottom of my image.

muraong wrote:

Is it Fake or not??

Fake...see Identify legitimate emails from the App Store or iTunes Store for details.

It is a phishing attempt. Do not respond. Do not divulge any personal or financial information. You can use the address below to forward the suspect email message, as an attachment to Apple.

reportphishing@apple.com

If you provided your Apple ID and password at the bogus site, change your password immediately.

  Change your Apple ID password - Apple Support