Event.command

Description

Is Command/Windows key held down? (Read Only)

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

using UnityEngine;

public class Example : MonoBehaviour
{
    // Prints Command/Windows key was pressed depending the
    // platform this script is being run.
    void OnGUI()
    {
        Event e = Event.current;
        if (e.command)
        {
            if (Application.platform == RuntimePlatform.OSXEditor)
            {
                Debug.Log("Command key was pressed");
            }
            else if (Application.platform == RuntimePlatform.WindowsEditor)
            {
                Debug.Log("Windows Key was pressed!");
            }
        }
    }
}