How do I know if someone has hacked into my iPhone?

Technically… If they’re any good at it, you don’t.

Most of these cases are not hacks, though.

Most of us are not worth the very expensive tools used for this, either.

Most of these threads are either compromised passwords or re-used passwords, or folks that know the local passcode or such, failure to use two-factor and getting phished, or confusion or concern over the contents of (normal) logs and (normal) telemetry data, failure to stay updated to current, and some other cases will involve gaslighting by someone.

To verify your security, start with the information Apple provides:

https://help.apple.com/pdf/personal-safety/en_US/personal-safety-user-guide.pdf

If you’re an investigative journalist, dissident or activist, seriously rich, senior in government or industry, or with access to classified or sensitive or financial data, the risk calculations here can and will shift.