Event.alt

Description

Is Alt/Option key held down? (Read Only)

On Windows, this returns true if any Alt key is held down.
On Mac, this returns true if any Option key is held down.

// Prints Option or Alt key was pressed depending the
// platform this script is being run.
function OnGUI() {
    var e : Event = Event.current;
    if (e.alt) {
        if(Application.platform == RuntimePlatform.OSXEditor) {
            Debug.Log("Option key was pressed");
        } else if( Application.platform == RuntimePlatform.WindowsEditor) {
            Debug.Log("Alt Key was pressed!");
        }
    }
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void OnGUI() {
        Event e = Event.current;
        if (e.alt)
            if (Application.platform == RuntimePlatform.OSXEditor)
                Debug.Log("Option key was pressed");
            else
                if (Application.platform == RuntimePlatform.WindowsEditor)
                    Debug.Log("Alt Key was pressed!");
                
        
    }
}