How would I know if my iPhone had a virus?

Do a spotlight search for the word "Cydia"

If you jailbroke it yourself you would know, to have someone else do it you would have had to given them the phone for about an hour and provided them with your passcode, Apple ID and password.

For one thing, it's not easy at all the jailbreak an iPhone. If you did it, you'd certainly know you did it. Unless as deggie suggests you gave your phone to someone and provided them your Passcode, Apple ID and Apple ID Password.

Unless your phone is jailbroken, there are no virus you can get. If you are having an issue, describe it fully so we can better advise you.

Jailbreaking means, “unauthorized modification,” of iPhone’s iOS, or software. If you don’t what jailbreak means, most likely your iPhone is not jailbroken.

What makes you believe your iPhone could have a virus?

Do you see something happening that makes you concerned? If yes, please describe. We might help.

It is a scam. They are trying to rip you off.

Well, it didn't actually come from Apple Security. That's how people get scammed.