Selectable.IsInteractable

public bool IsInteractable ();

Description

UI.Selectable.IsInteractable.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; // required when using UI elements in scripts

public class Example : MonoBehaviour
{
    public Button startButton;

    void Update()
    {
        if (!startButton.IsInteractable())
        {
            Debug.Log("Start Button has been Disabled");
        }
    }
}