How to check if user is authenticated in Razor pages of .Net Core 2.0

I've always used this option.

 private readonly SignInManager<IdentityUser> _signInManager;

        public HomeController(SignInManager<IdentityUser> signInManager)
        {
            _signInManager = signInManager;
        }

        public IActionResult Index()
        {
            if (_signInManager.IsSignedIn(User)) //verify if it's logged
            {
                return LocalRedirect("~/Page");
            }
            return View();
        }

Hope it helps someone!

Edit: David is right of course.

Just check if User or HttpContext.User.Identity.IsAuthenticated is true or not.

@if(!User.Identity.IsAuthenticated) 
{
    ...
}