How do I check my iPhone to see if it’s been hacked?

Unless you have given your phone to someone with evil knowledge for an hour or so and also provided them with your Apple ID and Password, your phone is not hacked.

The message that you saw and others like that you will see in the future are scams about 99.99% of the time that are simply trying to trick you into buying useless software that will do nothing to help your phone, but it will help to line the pockets of the guys who are selling the junk software.

Delete the message and move forward.

Any such popup, web page, email or message is a scam. Delete and ignore.