How do I tell if an email from apple is legitimate?

Strictly using the email address shown will not be helpful in determining authenticity as they are easily spoofed. You really need to review the entire email. On a Mac/PC, you can sometimes gain a little more information about the sender by viewing the Raw Header of the email, but usually the content will raise the biggest red flag. This article may be helpful in identifying and reporting phishing/scam emails.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support