// Prints a not used ID that can be assigned to a control
    function OnGUI() {
        // Gets a ID for a control that cannot receive keyboard focus (A button)
        Debug.Log("Available id: " + GUIUtility.GetControlID(FocusType.Passive));
    }

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnGUI() {
        Debug.Log("Available id: " + GUIUtility.GetControlID(FocusType.Passive));
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnGUI():
        Debug.Log(('Available id: ' + GUIUtility.GetControlID(FocusType.Passive)))