Event.alt

説明

「 Alt/Option 」キーが押されたかどうか（読み取り専用）

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!");
            }
        }
    }
}