Know if layer is in use

If anyone is interested, here's a piece of code that logs all unused layers. You have to execute this per scene though.

     public static void CheckUsedLayers()
     {
         Debug.Log("Check used layers");
         Dictionary<string, int> layerCount = new Dictionary<string, int>();
         List<GameObject> gameObjects = FindObjectsOfType<GameObject>().ToList();
 
         // iterate objects and save to dictionary
         for (int i = 0; i < gameObjects.Count; ++i)
         {
             string layerName = LayerMask.LayerToName(gameObjects[i].layer);
             if (layerCount.ContainsKey(layerName))
             {
                 layerCount[layerName]++;
             }
             else
             {
                 layerCount.Add(layerName, 1);
             }
         }
 
         // log to console
         foreach (KeyValuePair<string, int> entry in layerCount)
         {
             Debug.Log(entry.Key + ": " + entry.Value);
         }
 
         // unused layers
         List<string> layerNames = new List<string>();
         for (int i = 8; i <= 31; i++) //user defined layers start with layer 8 and unity supports 31 layers
         {
             var layerN = LayerMask.LayerToName(i); //get the name of the layer
             if (layerN.Length > 0) //only add the layer if it has been named (comment this line out if you want every layer)
                 layerNames.Add(layerN);
         }
 
         List<string> listOfKeys = layerCount.Keys.ToList();
         List<string> unusedLayers = layerNames.Except(listOfKeys).ToList();
         string joined = string.Join(", ", unusedLayers);
         Scene scene = SceneManager.GetActiveScene();
         Debug.Log("Unused layers in " + scene.name + ": " + joined);
 
         Debug.Log("Check used layers done");
     }