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;
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!");
                
        
    }
}