Loop through the fonts on iOS to find the internal name as described @ http://bit.ly/20yzZ7i

CustomFontsAppDelegate.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Foundation;
using UIKit;

namespace CustomXamFormsFont.iOS
{
    [Register("AppDelegate")]
    public partial class AppDelegate : global::Xamarin.Forms.Platform.iOS.FormsApplicationDelegate
    {
        public override bool FinishedLaunching(UIApplication app, NSDictionary options)
        {
            global::Xamarin.Forms.Forms.Init();

            LoadApplication(new App());
            
            // Lets log them to our output window.
            LogFonts();

            return base.FinishedLaunching(app, options);
        }

        /// <summary>
        /// Logs the installed fonts to the debug window.
        /// </summary>
        private void LogFonts()
        {
            foreach (NSString family in UIFont.FamilyNames)
            {
                foreach (NSString font in UIFont.FontNamesForFamilyName(family))
                {
                    Console.WriteLine(@"{0}", font);
                }
            }
        }
    }
}