PointerEventData.pointerId

Description

Identification of the pointer.

When using a mouse the pointerId returns -1, -2, or -3. These are the left, right and center mouse buttons respectively. When using touchscreen on mobile versions (such as iPad, iPhone, or Android), touches go from 0 up to however many touches the device supports.

using UnityEngine;
using UnityEngine.EventSystems;

/*To use the OnPointerDown, you must inherit from the IPointerDownHandler
and declare the function as public*/
public class Test : MonoBehaviour, IPointerDownHandler
{
    /*Called whenever a mouse click or touch screen tap is registered
    on the UI object this script is attached to.*/
    public void OnPointerDown(PointerEventData eventData)
    {
        int clickID = eventData.pointerId;

        if (clickID == -1)
        {
            Debug.Log("Left mouse click registered");
        }
        else if (clickID == -2)
        {
            Debug.Log("Right mouse click registered");
        }
        else if (clickID == -3)
        {
            Debug.Log("Center mouse click registered");
        }
        else if (clickID == 0)
        {
            Debug.Log("Single tap registered");
        }
        else if (clickID == 1)
        {
            Debug.Log("Double tap registered");
        }
        else if (clickID == 2)
        {
            Debug.Log("Triple tap registered");
        }
    }
}

using UnityEngine;
using UnityEngine.EventSystems;

/*To use the OnPointerDown, you must inherit from the IPointerDownHandler
and declare the function as public*/
public class Test : MonoBehaviour, IPointerDownHandler
{
    /*Called whenever a mouse click or touch screen tap is registered
    on the UI object this script is attached to.*/
    public void OnPointerDown(PointerEventData eventData)
    {
        switch (eventData.pointerId)
        {
            case -1:
                Debug.Log("Left mouse click registered");
                break;
            case -2:
                Debug.Log("Right mouse click registered");
                break;
            case -3:
                Debug.Log("Center mouse click registered");
                break;
            case 0:
                Debug.Log("Single tap registered");
                break;
            case 1:
                Debug.Log("Double tap registered");
                break;
            case 2:
                Debug.Log("Triple tap registered");
                break;
        }
    }
}