Part of answer for http://answers.unity3d.com/questions/878083/detect-play-mode-about-to-be-entered.html

ApplicationState.cs

using System;
using UnityEngine;

public static class ApplicationState
{
	/// <summary>	
	/// Is editor either currently in play mode, or about to switch to it?
	/// </summary>
	public static bool isPlayingOrWillChangePlaymode { get { return Application.isEditor ? InternalEditorState.isPlayingOrWillChangePlaymode : true; } }
	/// <summary>	
	/// Is editor currently paused?
	/// </summary>
	public static bool isPaused { get { return Application.isEditor ? InternalEditorState.isPaused : false; } }
	/// <summary>	
	/// Is editor currently compiling scripts?
	/// </summary>
	public static bool isCompiling { get { return Application.isEditor ? InternalEditorState.isCompiling : false; } }
	/// <summary>	
	/// This property is true if the Editor is currently refreshing the AssetDatabase.
	/// During this time, the editor checks to see if any files have changed, whether
	/// they need to be reimported, and reimports them.
	/// </summary>
	public static bool isUpdating { get { return Application.isEditor ? InternalEditorState.isUpdating : false; } }
	/// <summary>	
	/// Is editor currently connected to Unity Remote 4 client app.
	/// </summary>
	public static bool isRemoteConnected { get { return Application.isEditor ? InternalEditorState.isRemoteConnected : false; } }
	/// <summary>	
	/// Returns the path to the Unity editor application.
	/// </summary>
	public static string applicationPath { get { return Application.isEditor ? InternalEditorState.applicationPath : AppDomain.CurrentDomain.BaseDirectory; } }
	/// <summary>	
	/// The time since the editor was started.
	/// </summary>
	public static double timeSinceStartup { get { return Application.isEditor ? InternalEditorState.timeSinceStartup : Time.realtimeSinceStartup; } }
	/// <summary>	
	/// Is editor currently in play mode?
	/// </summary>
	public static bool isPlaying { get { return Application.isEditor ? InternalEditorState.isPlaying : true; } }
	/// <summary>	
	/// Path to the Unity editor contents folder.
	/// </summary>
	public static string applicationContentsPath { get { return Application.isEditor ? InternalEditorState.applicationContentsPath : Application.dataPath; } }

	public static bool isAboutToEnterPlayMode { get { return Application.isEditor ? !isPlaying && timeSinceStartup == 0 : false; } }

	public static string ToDebugString()
	{
		return
			"Application.isEditor = " + Application.isEditor + Environment.NewLine +
			"isPlayingOrWillChangePlaymode = " + isPlayingOrWillChangePlaymode + Environment.NewLine +
			"isPaused = " + isPaused + Environment.NewLine +
			"isCompiling = " + isCompiling + Environment.NewLine +
			"isUpdating = " + isUpdating + Environment.NewLine +
			"isRemoteConnected = " + isRemoteConnected + Environment.NewLine +
			"applicationPath = " + applicationPath + Environment.NewLine +
			"timeSinceStartup = " + timeSinceStartup + Environment.NewLine +
			"isPlaying = " + isPlaying + Environment.NewLine +
			"applicationContentsPath = " + applicationContentsPath + Environment.NewLine +
			"isAboutToEnterPlayMode = " + isAboutToEnterPlayMode + Environment.NewLine;
	}

	public static class InternalEditorState
	{
		public static bool isPlayingOrWillChangePlaymode;
		public static bool isPaused;
		public static bool isCompiling;
		public static bool isUpdating;
		public static bool isRemoteConnected;
		public static string applicationPath;
		public static double timeSinceStartup;
		public static bool isPlaying;
		public static string applicationContentsPath;

		public static string ToDebugString()
		{
			return
				"isPlayingOrWillChangePlaymode = " + isPlayingOrWillChangePlaymode + Environment.NewLine +
				"isPaused = " + isPaused + Environment.NewLine +
				"isCompiling = " + isCompiling + Environment.NewLine +
				"isUpdating = " + isUpdating + Environment.NewLine +
				"isRemoteConnected = " + isRemoteConnected + Environment.NewLine +
				"applicationPath = " + applicationPath + Environment.NewLine +
				"timeSinceStartup = " + timeSinceStartup + Environment.NewLine +
				"isPlaying = " + isPlaying + Environment.NewLine +
				"applicationContentsPath = " + applicationContentsPath + Environment.NewLine;
		}
	}

}

Editor UpdateApplicationState.cs

using UnityEditor;

[InitializeOnLoad]
public class UpdateApplicationState  {

	static UpdateApplicationState()
	{
		EditorApplication.update += Update;
		EditorApplication.playmodeStateChanged += PlaymodeStateChanged;
	}

	static void Update ()
	{
		ApplicationState.InternalEditorState.isPlayingOrWillChangePlaymode = EditorApplication.isPlayingOrWillChangePlaymode;
		ApplicationState.InternalEditorState.isPaused = EditorApplication.isPaused;
		ApplicationState.InternalEditorState.isCompiling = EditorApplication.isCompiling;
		ApplicationState.InternalEditorState.isUpdating = EditorApplication.isUpdating;
		ApplicationState.InternalEditorState.isRemoteConnected = EditorApplication.isRemoteConnected;
		ApplicationState.InternalEditorState.applicationPath = EditorApplication.applicationPath;
		ApplicationState.InternalEditorState.timeSinceStartup = EditorApplication.timeSinceStartup;
		ApplicationState.InternalEditorState.isPlaying = EditorApplication.isPlaying;
		ApplicationState.InternalEditorState.applicationContentsPath = EditorApplication.applicationContentsPath;
	}

	static void PlaymodeStateChanged()
	{
		Update();
	}
}