It just hit me from a conversation with my mother. It's not how well you can recite the Bible. It's not how eloquent you can convince people of God's sovereignty and election. It's not how astonish you can react to God's works. It's not how obedient you are of the statutes of God.
It's how much joy you find in the Lord in your affliction and suffering. In fact, inverse-proportionally (the more suffering the more joy). This is not some fake joy because fakeness won't last and is easily perceived to be vain. Paul said it. Peter said it. As the Lord in His own example taught it. Such is someone who has truly been admitted before the bosom of the Lord. A true believer indeed, if not professed to be reformed.