How do I find out if my phone is compromised?

It would be helpful for you to explain why you think your phone is compromised. If you received a text message, email message, or website pop up stating that your phone is compromised then it is a scam, ignore it and delete it.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Have you jailbroken your phone? Has someone else had access to it for an extended period of time while it was unlocked? Are you a high-profile activist/politician/spy/criminal someone would be willing to spend millions of dollars on to get your information? If the answer to those questions is no, your phone has not been compromised.

That doesn't mean your Apple ID can't be compromised.

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

But as Revianmac suggested, knowing why you think this, what specific things you're seeing, would help people give you better answers.

Yes. I chose Apple after being hacked on a phone connected to Google One. The first thing you must do is not bring compromised accounts or their apps onto your secure apple device. Be aware that sometimes the best option is to delete compromised accts and start fresh. Hope that info points you in the right direction when assessing issues you are worried about