GUIUtility.keyboardControl

설명

The controlID of the control that has keyboard focus.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    private string str = "A String!";
    void OnGUI() {
        str = GUILayout.TextField(str, 10);
        Debug.Log("id: " + GUIUtility.keyboardControl);
    }
}