How will I notice if my iPhone is Hacked

That message/email/website popup is a scam. If you clicked on that link and did not enter any of your credentials then you are fine. If you entered Username(s) or Password(s) then change the password of whatever account information you entered.

It is nearly impossible to hack your iPhone due to Apple's closed infrastructure unless your iPhone has been jailbroken.

More information on Scams below-

Beware of ➔ Phishing & Other Suspicious Emails 

Know how to ➔ Identify legitimate emails from the App Store or iTunes Store 

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams ➔ Use these tips to avoid scams and learn what to do if you think your Apple ID has been compromised.

Axel F.

Scam! Delete and ignore. Do not reply!

Scam message. A non-jailbroken phone cannot be hacked.