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')