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.

using UnityEngine;

public class Example : MonoBehaviour
{
    // Prints Option or Alt key was pressed depending the
    // platform this script is being run.
    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!");
            }
        }
    }
}