I've heard it said that discernment is not being able to tell what is right from what is wrong, it's being able to tell what is right from what is almost right.
Do they portray themselves as right? Or are they "almost right" - or sound almost right?
One of the core doctrines of Christianity is the doctrine of the Trinity. If you can really nail a Mormon down...they deny it.
God as an eternal being - uncreated. They don't believe that.