How do I determine if my iPhone Apple Account has been compromised?

See this on how to determine if your apple account is compromised

• Apple notifies you (notification or email) about account activity that you don't recognize (for example, if your Apple Account was signed in to on a device that you don't recognize or your password was changed but you didn't change it).
• You receive a two-factor authentication code (either on a trusted device or via text message) that you didn't request.
• You notice unusual activity, such as messages that you didn't send, deleted items that you didn’t delete, account details that you didn't change or don't recognize, trusted devices that you didn't add or don't recognize, or purchase activity that you don't recognize.
• Your password no longer works.
• Your device was locked or placed in Lost Mode by someone other than you.

Learn how to recognize and avoid social engineering schemes, including phishing scams

Learn what to do if you see an unfamiliar iTunes Store or App Store charge

See this link for further reference

If you think your Apple Account has been compromised - Apple Support