How to know if your iPhone has viruses

You don't.....unless....you let someone use your phone for an hour or two and also provided them with your Apple ID and Password.

If you really do suspect something is amiss.....run Safety Check on your phone.

How Safety Check on iPhone works to keep you safe - Apple Support

There are no known viruses in the wild that can infect an iPhone that has not been jailbroken.

ohio289 wrote:

But I didn’t give it to someone

Then you will need to provide details as to what is happening to make you suspect a virus.