How do I know if my Apple ID has been hacked

According to: If you think your Apple ID has been compromised - Apple Support

"Signs your Apple ID has been compromised

Your Apple ID might be compromised if you receive an account notification from Apple for a change you didn't make, or if you notice account details or changes you don’t recognize. For example:

• You receive an email or notification that your Apple ID was used to sign in to a device you don't recognize or did not sign in to recently (for example, "Your Apple ID was used to sign in to iCloud on a Windows PC").
• You receive a confirmation email from Apple that your Apple ID password was changed or your account information was updated, but you don’t remember making any changes.
• Your device was locked or placed in Lost Mode by someone other than you.
• You see messages you didn't send, or items you didn’t delete.
• You see charges or notices for purchases that you didn't make. Learn what to do if you see an unfamiliar iTunes Store or App Store charge on your credit or debit card statement.
• Your password no longer works, or it might have been changed or locked.
• You don't recognize some or all of your account details."

If tis is in reference to a phone call you have received, see:

Apple does not make such calls.

All such calls are scams from criminals attempting to steal your personal and financial information. The callers will often use number spoofing to pretend they are calling from a legitimate business. The correct course of action is to hang up on them, repeatedly if needed. 

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support