// Detects if the shift key was pressed
function OnGUI() {
    var e : Event = Event.current;
    if (e.shift) {
        Debug.Log("Shift was pressed :O");
    }
}

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnGUI():
        e as Event = Event.current
        if e.shift:
            Debug.Log('Shift was pressed :O')