ソースコード

using System.Collections.Generic;
using UnityEngine.SceneManagement;

namespace Kogane
{
    public static class KoganeSceneManager
    {
        public static bool IsLoaded( string sceneName )
        {
            var sceneCount = SceneManager.sceneCount;

            for ( var i = 0; i < sceneCount; i++ )
            {
                var scene = SceneManager.GetSceneAt( i );

                if ( scene.name == sceneName && scene.isLoaded ) return true;
            }

            return false;
        }
    }
}

使用例

Debug.Log( KoganeSceneManager.IsLoaded( "SampleScene" ) );