Event.isMouse  

// Detects any mouse event
function OnGUI() {
var e : Event = Event.current;
if (e.isMouse){
Debug.Log("Detected a mouse event!");
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnGUI() {
Event e = Event.current;
if (e.isMouse)
Debug.Log("Detected a mouse event!");

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnGUI():
e as Event = Event.current
if e.isMouse:
Debug.Log('Detected a mouse event!')