// 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!')