Is there a way of knowing if it's really Apple emailing you? I have an invoice sent to my email that I don't think is right and might be fraud

Identifying fraudulent "phishing" email

Phishing & Other Suspicious Emails

An invoice for what? What have you bought recently from Apple? If you have purchased something recently, check it against your purchase history in the iTunes store from a computer running iTunes (that way you wil see your detailed purchase history in your account when you log in). Or check it against your credit card or billing statement. Or simply call Apple directly and ask them about it. The contact information can be found by clicking on the "contact us" link at the bottom right of this page.

If you have any doubts about it, do NOT click any links in the email - just check the information independently yourself against your accounts and/or records.