Unity3D ScenePreview inspector: http://diegogiacomelli.com.br/unity3d-scenepreview-inspector/

ScenePreview.cs

using UnityEngine;
using UnityEditor;
using UnityEngine.SceneManagement;
using System.Linq;

/// <summary>
/// Scene preview.
/// https://diegogiacomelli.com.br/unity3d-scenepreview-inspector/
/// </summary>
[CustomEditor(typeof(SceneAsset))]
[CanEditMultipleObjects]
public class ScenePreview : Editor
{
    // Change this to a folder in your project. 
    // Maybe the folder where your scenes are located. Remember to create a subfolder called "Resources" inside of it.
    const string PreviewFolders = "_scenes";
    const float EditorMargin = 50;
    const float PreviewMargin = 5;
    static bool _shouldRefreshDatabase;

    [RuntimeInitializeOnLoadMethod]
    public static void CaptureScreenshot()
    {
        var previewPath = GetPreviewPath(SceneManager.GetActiveScene().name);
        Debug.LogFormat("Saving scene preview at {0}", previewPath);
        ScreenCapture.CaptureScreenshot(previewPath);
        Debug.LogFormat("Scene preview saved at {0}", previewPath);

        _shouldRefreshDatabase = true;
    }

    public override void OnInspectorGUI()
    {        
        if (_shouldRefreshDatabase)
        {
            AssetDatabase.Refresh(ImportAssetOptions.ForceUpdate);
            _shouldRefreshDatabase = false;
        }

        var sceneNames = targets.Select(t => ((SceneAsset)t).name).OrderBy(n => n).ToArray();

        var previewsCount = sceneNames.Length;
        var previewWidth = Screen.width;
        var previewHeight = (Screen.height - EditorMargin * 2 - (PreviewMargin * previewsCount)) / previewsCount;

        for (int i = 0; i < sceneNames.Length; i++)
        {
            DrawPreview(i, sceneNames[i], previewWidth, previewHeight);
        }
    }

    void DrawPreview(int index, string sceneName, float width, float height)
    {
        var previewPath = GetPreviewPath(sceneName);
        var preview = Resources.Load(sceneName) as Texture;

        if (preview == null)
        {
            EditorGUILayout.HelpBox(string.Format(
                "There is no image preview for scene {0} at {1}. Please play the scene on editor and image preview will be captured automatically.",
                sceneName,
                previewPath),
                MessageType.Info);
        }
        else
        {
            GUI.DrawTexture(new Rect(index, EditorMargin + index * (height + PreviewMargin), width, height), preview, ScaleMode.ScaleToFit);
        }
    }

    static string GetPreviewPath(string sceneName)
    {
        return string.Format("{0}/{1}/Resources/{2}.png", Application.dataPath, PreviewFolders, sceneName);
    }
}