How to tell if an iPhone is being monitored?

See the three (3x) checklists at the end of this Personal Safety Guide:

Personal Safety User Guide – Apple Support (AU)

(Click on Table of Contents)

Then, login to https://appleid.apple.com/

Review EVERYTHING - especially your “Devices” and “Trusted Numbers” - deleting and/or updating as appropriate.

Lastly, use the following guide:

Use the built-in privacy and security protections of iPhone – Apple Support (AU)

Here’s one more thing to check→Checklist: If you want to see if anyone else has access to your device or accounts - Apple Support