function OnGUI() {
GUILayout.Button( "My button" );
if(Event.current.type == EventType.Repaint &&
GUILayoutUtility.GetLastRect().Contains(Event.current.mousePosition )) {
GUILayout.Label( "Mouse over!" );
} else {
GUILayout.Label( "Mouse somewhere else" );
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnGUI() {
GUILayout.Button("My button");
if (Event.current.type == EventType.Repaint && GUILayoutUtility.GetLastRect().Contains(Event.current.mousePosition))
GUILayout.Label("Mouse over!");
else
GUILayout.Label("Mouse somewhere else");
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnGUI():
GUILayout.Button('My button')
if (Event.current.type == EventType.Repaint) and GUILayoutUtility.GetLastRect().Contains(Event.current.mousePosition):
GUILayout.Label('Mouse over!')
else:
GUILayout.Label('Mouse somewhere else')