Determine built in editor menu items

Here is a script that will go through all the Editor scripts and parse out the shortcuts. It creates a dictionary of command names to their keyboard shortcuts if specified. It includes a Help menu item which will write them to the console.

 using UnityEngine;
 using UnityEditor;
 using System.IO;
 using System.Collections.Generic;
 using System.Text.RegularExpressions;
 using System.Linq;
 
 static class FindAllScriptShortcuts
 {
     static private Regex MenuItemRegex = new Regex(@"\s*[@[]\s*MenuItem\s*\(\s*""(?<command>[^_%&#""]+)\s*(?<shortcut>[_%&#]+[^\s]|)\s*""\)\s*]*");
     
     [MenuItem(@"Help/Display Shortcuts")]
     static void DisplayShortcuts()
     {
         var shortcuts = GetScriptShortcuts();
         foreach(var shortcut in shortcuts)
         {
             Debug.Log(shortcut.Key + " => " + shortcut.Value);
         }
     }
     
     static public Dictionary<string,string> GetScriptShortcuts()
     {
         var shortcuts = new Dictionary<string,string>();
 
         // Get the path to the editor scripts
         var path = Path.Combine (Application.dataPath, "Editor");
         // If such a path exists go through the files and find the shortcuts
         if(Directory.Exists(path))
         {
             // Get all script files
             var files = Directory.GetFiles (path, "*.*", SearchOption.AllDirectories).Where (p => new[]{".cs", ".js", ".boo"}.Contains (Path.GetExtension(p)));
             foreach(var file in files)
             {
                 // Find all the command shortcuts in the file
                 foreach(Match shortcut in MenuItemRegex.Matches(File.ReadAllText(file)))
                 {
                     // Add the found shortcuts into our collection
                     shortcuts[shortcut.Groups["command"].Captures[0].Value] = shortcut.Groups["shortcut"].Captures[0].Value;
                 }
             }
         }
         return shortcuts;
     }
 }

[Edited: removed list of version specific shortcuts]

To get MenuItem shortcuts out of an assembly you can use Reflection. The example below reflects on the UnityEditor.dll, which is where MenuItem is defined. You could do this for all assemblies of interest, and would be Unity version agnostic.

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 using System.Reflection;
 
 public class MenuItems  {
     
     [MenuItem("Help/Display Shortcuts")]
     static void GetMenuItems()
     {
         var assembly = Assembly.GetAssembly(typeof(MenuItem));
         foreach(var type in assembly.GetTypes())
         {
             var methods = type.GetMethods();
             foreach(var method in methods)
             {
                 var attributes = method.GetCustomAttributes(false);
                 foreach(var attribute in attributes)
                 {
                     var menuItem = attribute as MenuItem;
                     if(menuItem != null)
                     {
                         Debug.Log (menuItem.menuItem);
                     }
                 }
             }
         }
     }
 }
 

Cheers!

[1]: http://docs.unity3d.com/Documentation/ScriptReference/MenuItem.html