A serializable struct for referencing and, at runtime, handle scenes.

RuntimeSceneReference.cs

#if UNITY_EDITOR
using UnityEditor;
#endif
using UnityEngine;
using UnityEngine.SceneManagement;

[System.Serializable]
public struct RuntimeSceneReference : ISerializationCallbackReceiver
{
#if UNITY_EDITOR
    internal static class PropertyNames
    {
        internal const string sceneAsset = nameof(RuntimeSceneReference.sceneAsset);
    }
#endif

    [SerializeField, HideInInspector]
    private string sceneName;

#if UNITY_EDITOR
    [SerializeField]
    private SceneAsset sceneAsset;
#endif

    /// <summary>
    /// Loads the scene if it isn't loaded yet.
    /// </summary>
    public void LoadSingle(LoadSceneMode mode = LoadSceneMode.Single)
    {
        if (SceneIsLoaded()) return;

        SceneManager.LoadScene(sceneName, mode);
    }

    /// <summary>
    /// Loads the scene if it isn't loaded yet.
    /// </summary>
    public AsyncOperation LoadSingleAsync(LoadSceneMode mode = LoadSceneMode.Single)
    {
        if (SceneIsLoaded()) return null;

        return SceneManager.LoadSceneAsync(sceneName, mode);
    }

    public AsyncOperation Unload()
    {
        if (!SceneIsLoaded()) return null;

        return SceneManager.UnloadSceneAsync(sceneName);
    }

    private bool SceneIsLoaded()
    {
        return SceneManager.GetSceneByName(sceneName).isLoaded;
    }

    public bool SetAsActiveScene()
    {
        var scene = SceneManager.GetSceneByName(sceneName);
        return SceneManager.SetActiveScene(scene);
    }

    void ISerializationCallbackReceiver.OnAfterDeserialize()
    {

    }

    void ISerializationCallbackReceiver.OnBeforeSerialize()
    {
#if UNITY_EDITOR
        sceneName = sceneAsset != null ? sceneAsset.name : null;
#endif
    }
}

RuntimeSceneReferenceDrawer.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(RuntimeSceneReference))]
public class RuntimeSceneReferenceDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        property = property.FindPropertyRelative(RuntimeSceneReference.PropertyNames.sceneAsset);

        var sceneAsset = (SceneAsset)property.objectReferenceValue;
        if (sceneAsset != null)
        {
            var isInBuildList = IsInBuildList(sceneAsset);
            if (!isInBuildList)
            {
                const int labelRectWidth = 100;

                GUI.color = new Color(1f, 0.4f, 0.4f, 1f);
                var labelRect = new Rect(position.x + position.width - labelRectWidth, position.y, labelRectWidth, position.height);
                GUI.Box(labelRect, "Not in Build List!");

                position.xMax -= labelRectWidth + 3;
            }
        }

        EditorGUI.PropertyField(position, property, label);
        GUI.color = Color.white;
    }

    private static bool IsInBuildList(SceneAsset sceneAsset)
    {
        foreach (var scene in EditorBuildSettings.scenes)
        {
            if (scene.path == AssetDatabase.GetAssetPath(sceneAsset) && scene.enabled)
            {
                return true;
            }
        }

        return false;
    }
}