Scripting Reference
Input.GetMouseButton

static function GetMouseButton (button : int) : boolean

Description

Returns whether the given mouse button is held down.

button values are 0 for left button, 1 for right button, 2 for the middle button.

JavaScript 
    // Detects clicks from the mouse and prints a message
    // depending on the click detected.
    function Update() {
        if(Input.GetMouseButton(0))
            Debug.Log("Pressed left click.");
        if(Input.GetMouseButton(1))
            Debug.Log("Pressed right click.");
        if(Input.GetMouseButton(2))
            Debug.Log("Pressed middle click.");
    }
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Update() {
        if (Input.GetMouseButton(0))
            Debug.Log("Pressed left click.");
        
        if (Input.GetMouseButton(1))
            Debug.Log("Pressed right click.");
        
        if (Input.GetMouseButton(2))
            Debug.Log("Pressed middle click.");
        
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Update():
        if Input.GetMouseButton(0):
            Debug.Log('Pressed left click.')
        if Input.GetMouseButton(1):
            Debug.Log('Pressed right click.')
        if Input.GetMouseButton(2):
            Debug.Log('Pressed middle click.')