// Detect which mouse button is currently pressed
// and print it.
function OnGUI() {
    var e : Event = Event.current;
    if(e.button == 0 && e.isMouse){
        Debug.Log("Left Click");
    } else if(e.button == 1) {
        Debug.Log("Right Click");
    } else if (e.button == 2) {
        Debug.Log("Middle Click");    
    } else if (e.button > 2) {
        Debug.Log("Another button in the mouse clicked");
    }
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnGUI() {
        Event e = Event.current;
        if (e.button == 0 && e.isMouse)
            Debug.Log("Left Click");
        else
            if (e.button == 1)
                Debug.Log("Right Click");
            else
                if (e.button == 2)
                    Debug.Log("Middle Click");
                else
                    if (e.button > 2)
                        Debug.Log("Another button in the mouse clicked");
                    
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnGUI():
        e as Event = Event.current
        if (e.button == 0) and e.isMouse:
            Debug.Log('Left Click')
        elif e.button == 1:
            Debug.Log('Right Click')
        elif e.button == 2:
            Debug.Log('Middle Click')
        elif e.button > 2:
            Debug.Log('Another button in the mouse clicked')