IAds Banner help. If IAds isn't showing show admob.

I'm using this code but I don't know how to check if the ad is showing. If the ad isn't showing i wan't it to show an admob ad and if the IAds start to show, remove the admob and show the IAds.

 using UnityEngine;
 using System.Collections;
 
 public class IAdScript : MonoBehaviour {
 
     private ADBannerView banner = null;
     void Start()
     {
         banner = new ADBannerView(ADBannerView.Type.Banner, ADBannerView.Layout.Bottom);
         ADBannerView.onBannerWasClicked += OnBannerClicked;
         ADBannerView.onBannerWasLoaded  += OnBannerLoaded;
     }
     void OnBannerClicked()
     {
         Debug.Log("Clicked!\n");
     }
     void OnBannerLoaded()
     {
         Debug.Log("Loaded!\n");
         banner.visible = true;
     }
 }