will my iPhone 12 tell me if its been compromised

You may get a message that says your Apple ID was used to sign in on another device, but you will never be asked to enter your Apple ID/Password. Also a message will never use the word "Hack".

Without any further information why you think your account has been compromised, there can be no specific advice to relate to your situation.

At anytime if you think your Apple ID has been compromised, then follow this Support Article:

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

Oochnyc wrote:

Will my iPhone 12 warn me that it’s been compromised or was that a hack?

If the miscreants are any good, no. Of course not. A big “hacked’ message tends to be counterproductive, after all. But the sorts of exploits most folks imagine happening are exceedingly expensive, and have been very much targeted at specific people, per all available information. For the rest of us, Apple ID compromises and passcode compromises and password re-use and such are the usual problems, when we do get in trouble.

So… why do you think you might be hacked, and what have you already done to potentially remedy the issues being experienced, and what happened?

Oochnyc Said:

"Will my iPhone 12 warn me that it’s been compromised or was that a hack?"

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iPhone Telling you It's Benn Compromised:

No. It wouldn't tell you that, as it is not part of the iOS.

About a Hacked iPhone:

Unfortunately, there is no way to determine if it has been hacked, unless you know that it has been hacked (officially called "jailbroken". And, the only way for an iPhone to have hacked is if it has been jailbroken.

So, what to do...

Reset to Factory Settings:

If you are so concerned, then reset your iPhone to factory settings, as if you were to sell it. Go here: What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support. Perform as instructed there, including the backup creation and iCloud logout.