I'm trying to develop a plugin which send a push notification to an app users, when a new post is published on a certain category. I'm trying with the hook save_post, but when the hook is fired, the post has no categories. I try in_category(2) function, but it always return false. I try wp_get_post_categories($post_id) but always resolve to no categories and the same with wp_get_post_terms($post_id, 'category')

I dont know if save_post hook is the correct, I think it's fired before the categories are asociated to the post.

How I know if the post is published in a certain category?