find iPhone owner

You don’t. The user has had the opportunity to display a message, at their choice. If the message doesn’t show when charged, there isn’t one. This information would be massively misused by abusive exes, thieves, and ne’er-do-wells. Turn it in to the folks in charge of maintaining where it was found, or to the local police if that’s an option. Otherwise, recycle it.

You don't.

Turn it into the lost and found or police.