Detect missing methods in scene UnityEvents.

MissingOnClickDetector.cs

/*
* Based on code found here: https://stackoverflow.com/questions/42784338/unity-missing-warning-when-button-has-missing-onclick/42788400?noredirect=1#comment91506017_42788400
*/
#if UNITY_EDITOR
using System;
using System.Reflection;
using UnityEngine;
using UnityEngine.UI;

public class MissingOnClickDetector : MonoBehaviour
{
    void Awake()
    {
        //Debug.Log("Class exist? " + classExist("ok.ButtonCallBackTest"));
        SearchForMissingOnClickFunctions();
    }

    void SearchForMissingOnClickFunctions()
    {
        //Find all Buttons in the scene including hiding ones
        Button[] allButtonScriptsInScene = Resources.FindObjectsOfTypeAll<Button>() as Button[];
        for (int i = 0; i < allButtonScriptsInScene.Length; i++)
        {
            DetectButtonError(allButtonScriptsInScene[i]);
        }
    }

    //Searches each registered onClick function in each class
    void DetectButtonError(Button button)
    {
        for (int i = 0; i < button.onClick.GetPersistentEventCount(); i++)
        {
            //Get the target class name
            UnityEngine.Object objectName = button.onClick.GetPersistentTarget(i);

            //Get the function name
            string methodName = button.onClick.GetPersistentMethodName(i); ;

            //////////////////////////////////////////////////////CHECK CLASS/SCRIPT EXISTANCE/////////////////////////////////////////

            //Check if the class that holds the function is null then exit if it is 
            if (objectName == null)
            {
                Debug.Log("<color=blue>Button \"" + button.gameObject.name +
                    "\" is missing the script that has the supposed button callback function. " +
                    "Please check if this script still exist or has been renamed</color>", button.gameObject);
                continue; //Don't run code below
            }

            //Get full target class name(including namespace)
            string objectFullNameWithNamespace = objectName.GetType().FullName;

            //Check if the class that holds the function exist then exit if it does not
            if (!ClassExist(objectFullNameWithNamespace))
            {
                Debug.Log("<color=blue>Button \"" + button.gameObject.name +
                     "\" is missing the script that has the supposed button callback function. " +
                     "Please check if this script still exist or has been renamed</color>", button.gameObject);
                continue; //Don't run code below
            }

            //////////////////////////////////////////////////////CHECK FUNCTION EXISTANCE/////////////////////////////////////////

            //Check if function Exist as public (the registered onClick function is ok if this returns true)
            if (FunctionExistAsPublicInTarget(objectName, methodName))
            {
                //No Need to Log if function exist
                //Debug.Log("<color=green>Function Exist</color>");
            }

            //Check if function Exist as private 
            else if (FunctionExistAsPrivateInTarget(objectName, methodName))
            {
                Debug.Log("<color=yellow>The registered Function \"" + methodName + "\" Exist as a private function. Please change \"" + methodName +
                    "\" function from the \"" + objectFullNameWithNamespace + "\" script to a public Access Modifier</color>", button.gameObject);
            }

            //Function does not even exist at-all
            else
            {
                Debug.Log("<color=red>The \"" + methodName + "\" function Does NOT Exist in the \"" + objectFullNameWithNamespace + "\" script</color>", button.gameObject);
            }
        }
    }

    //Checks if class exit or has been renamed
    bool ClassExist(string className)
    {
        Type myType = Type.GetType(className);
        return myType != null;
    }

    //Checks if functions exist as public function
    bool FunctionExistAsPublicInTarget(UnityEngine.Object target, string functionName)
    {
        try
        {
            Type type = target.GetType();
            MethodInfo targetinfo = type.GetMethod(functionName);
            return targetinfo != null;
        }
        catch (AmbiguousMatchException)
        {
            return true;
        }
    }

    //Checks if functions exist as private function
    bool FunctionExistAsPrivateInTarget(UnityEngine.Object target, string functionName)
    {
        Type type = target.GetType();
        MethodInfo targetinfo = type.GetMethod(functionName, BindingFlags.Instance | BindingFlags.NonPublic);
        return targetinfo != null;
    }
}
#endif

See this Stack Overflow page for more context about this code.