EventType.MouseDown

Description

Mouse button was pressed.

This event gets sent when any mouse button is pressed. Use Event.button to determine which button was pressed down.

Additional resources: EventType, Event.Use.

using UnityEngine;

public class Example : MonoBehaviour
{
    void OnGUI()
    {
        Event m_Event = Event.current;

        if (m_Event.type == EventType.MouseDown)
        {
            if (m_Event.pointerType == PointerType.Pen)     //Check if it's a pen event.
                Debug.Log("Pen Down.");
            else 
                Debug.Log("Mouse Down.");                   //If it's not a pen event, it's a mouse event. 
        }
    }
}