Event.shift

Description

Is Shift held down? (Read Only)

Returns true if any Shift key is held down.

using UnityEngine;

public class Example : MonoBehaviour
{
    // Detects if the shift key was pressed
    void OnGUI()
    {
        Event e = Event.current;
        if (e.shift)
        {
            Debug.Log("Shift was pressed :O");
        }
    }
}