Event.alt

설명

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;
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!");
                
        
    }
}