Input.GetMouseButton

Description

Returns whether the given mouse button is held down.

The button values are: 0 for the left button, 1 for the right button, 2 for the middle button. The return is true when the mouse button is pressed down, and false when released.

using UnityEngine;
using System.Collections;

// Detects clicks from the mouse and prints a message
// depending on the click detected.

public class ExampleClass : MonoBehaviour
{
    void Update()
    {
        if (Input.GetMouseButton(0))
        {
            Debug.Log("The left mouse button is being held down.");
        }

        if (Input.GetMouseButton(1))
        {
            Debug.Log("The right mouse button is being held down.");
        }

        if (Input.GetMouseButton(2))
        {
            Debug.Log("The middle mouse button is being held down.");
        }
    }
}