GUIUtility.hotControl

Description

The controlID of the current hot control.

The hot control is one that is temporarily active. When the user mousedown's on a button, it becomes hot.
No other controls are allowed to respond to mouse events while some other control is hot.
once the user mouseup's, the control sets hotControl to 0 in order to indicate that other controls can now respond to user input.

    // Click on the button to see the id

    function OnGUI() {
        GUILayout.Button("Press Me!");
        Debug.Log("id: " + GUIUtility.hotControl);
    }

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    void OnGUI() {
        GUILayout.Button("Press Me!");
        Debug.Log("id: " + GUIUtility.hotControl);
    }
}