Event.button  

// 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')