Simple editor window for checking lightmap in unity

UnityLightmapAssist.cs

#if UNITY_EDITOR
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

public class LightmapAssist : EditorWindow 
{
    [MenuItem("Window/Lighting/Lightmap Assistant")]
    public static void ShowLightmapAssistWnd()
    {
        EditorWindow.GetWindow(typeof(LightmapAssist), true, "LightmapAssistant");
    }

    void OnSelectionChange()
    {
        Repaint();
    }

    void OnDestroy()
    {
        if (mLightmapPreviewMaterial != null)
        {
            DestroyImmediate(mLightmapPreviewMaterial);
            mLightmapPreviewMaterial = null;
        }
    }

    public void OnGUI()
    {
        var go = Selection.activeGameObject;
        if (go == null)
        {
            return;
        }

        var renderer = go.GetComponent<MeshRenderer>();
        if (renderer == null || renderer.lightmapIndex == -1)
        {
            EditorGUILayout.LabelField(string.Format("{0} has no lightmap.", go.name));
            return;
        }

        var lightmap = LightmapSettings.lightmaps[renderer.lightmapIndex].lightmapColor;

        var path = AssetDatabase.GetAssetPath(lightmap);

        if (string.IsNullOrEmpty(path) == false)
        {
            EditorGUILayout.LabelField(string.Format("GameObject:{0},  Lightmap Path:{1},  LightMapIndex:{2}", go.name, path, renderer.lightmapIndex));
        }
        else
        {
            EditorGUILayout.LabelField(string.Format("GameObject:{0}", go.name));
        }

        DrawLightmapWithWireframe(EditorGUILayout.GetControlRect(false, GUILayout.ExpandWidth(true), GUILayout.ExpandHeight(true)), go.GetComponent<MeshFilter>().sharedMesh, 
            lightmap, renderer.lightmapScaleOffset);
    }

    private void DrawLightmapWithWireframe(Rect rect, Mesh mesh, Texture2D lightmap, Vector4 lightmapScaleOffset)
    {
        if (mLightmapPreviewMaterial == null)
        {
            mLightmapPreviewMaterial = new Material(Shader.Find("GUI/LightmapAssist Prievew"));
            mLightmapPreviewMaterial.hideFlags = HideFlags.HideAndDontSave;
        }

        mLightmapPreviewMaterial.SetVector("_Viewport", Vector4.zero);
        mLightmapPreviewMaterial.color = Color.white;
        mLightmapPreviewMaterial.SetVector("_MainTex_ST", new Vector4(1, 1, 0, 0));
        EditorGUI.DrawPreviewTexture(rect, lightmap, mLightmapPreviewMaterial, ScaleMode.StretchToFill);

        mLightmapPreviewMaterial.color = new Color32(104, 138, 188, 255);
        mLightmapPreviewMaterial.SetTexture("_MainTex", Texture2D.whiteTexture);
        mLightmapPreviewMaterial.SetVector("_MainTex_ST", lightmapScaleOffset);
        mLightmapPreviewMaterial.SetPass(0);

        var triangles = mesh.triangles;
        var vertices  = mesh.uv2.Length <= 0 ? mesh.uv : mesh.uv2;

        GL.wireframe = true;
        GL.PushMatrix();
        {
            GL.MultMatrix(Matrix4x4.TRS(new Vector3(rect.xMin, rect.yMin + rect.height, 0), Quaternion.identity, new Vector3(rect.width, -rect.height, 1) ));

            GL.Begin(GL.TRIANGLES);
            {
                for (int i = 0; i < triangles.Length; i++)
                {
                    GL.Vertex3(vertices[triangles[i]].x, vertices[triangles[i]].y, 0f);
                }
            }
            GL.End();
        }
        GL.PopMatrix();
        GL.wireframe = false;
    }

    private Material mLightmapPreviewMaterial;
}
#endif

UnityLightmapAssistPreview.shader

Shader "GUI/LightmapAssist Prievew" 
{
	Properties {
		_MainTex ("Sprite Texture", 2D) = "white" {}
		_Color ("Text Color", Color) = (1,1,1,1)
	}

	SubShader {

		Tags {
			"Queue"="Transparent"
			"IgnoreProjector"="True"
			"RenderType"="Transparent"
			"PreviewType"="Plane"
		}
		Lighting Off Cull Off ZTest Always ZWrite Off
		Blend SrcAlpha OneMinusSrcAlpha

		Pass {
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#include "UnityCG.cginc"

			struct appdata_t {
				float4 vertex : POSITION;
				fixed4 color : COLOR;
				float2 texcoord : TEXCOORD0;
			};

			struct v2f {
				float4 vertex : SV_POSITION;
				fixed4 color : COLOR;
				float2 texcoord : TEXCOORD0;
			};

			sampler2D _MainTex;
			uniform float4 _MainTex_ST;
			uniform fixed4 _Color;
			v2f vert (appdata_t v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(float4(v.vertex.xy * _MainTex_ST.xy + _MainTex_ST.zw, v.vertex.zw));
				o.color = v.color * _Color;
				o.texcoord = TRANSFORM_TEX(v.texcoord,_MainTex);
				return o;
			}

			fixed4 frag (v2f i) : SV_Target
			{
				fixed4 col = i.color * tex2D(_MainTex, i.texcoord);
				return col;
			}
			ENDCG
		}
	}
}