How to know if a relation has been updated on a given model in laravel?

I'm currently working on a Laravel application which is using Model events to perform data validation / synchronization in the database.

I've a table which contains important data. This table is updated on different Model updates. I use model events to handle that. It works on Model properties example :

<?php
Product::saved(function (Product $p) {
    $dirty = collect($p->getDirty());
    if ($dirty->has('ugr') || $dirty->has('ra')) {
        //Do some stuff here
    }
});

With that kind of logic I can restrict my "stuff" execution on specific model property update.

How can I make that kind of check on Product relations ?

I've a ManyToMany relationships accessible through the applications methods on Product, how can I know if the linked application list has changed since model loading ?

Thanks for your help !

via Chebli Mohamed