Yes, Richard!! We CAN know when a soul is truly converted--there are two key interconnected elements to true conversion, and we will get to this profound principle of the foundation of the gospel in Chapter 73, "Let Not Your Heart Be Troubled." It states on page 676, paragraph 4:
"When we live by faith on the Son of God, the fruits of the Spirit will be seen in our lives; not one will be missing."
We see two keys in light of Simon--first, he DID NOT live by faith on the Son of God; his interactions with Jesus had yet to kindle full faith in Christ as Messiah, and evidence of th doubt he cherished is seen in how he watched the way Christ treated Mary--and actually judged Christ in his heart as not being the Messiah, simply because Christ acted mercifully!
Secondly, Simon did not have Christ in His heart, so the fruits of the Spirit WERE NOT in his life without one missing.
But praise the Lord--there was hope for him!! He repented and turned entirely to Jesus--and received a new heart!!
Simon was touched by the kindness of Jesus in not openly rebuking him before the guests. He had not been treated as he desired Mary to be treated. He saw that Jesus did not wish to expose his guilt to others, but sought by a true statement of the case to convince his mind, and by pitying kindness to subdue his heart. Stern denunciation would have hardened Simon against repentance, but patient admonition convinced him of his error. He saw the magnitude of the debt which he owed his Lord. His pride was humbled, he repented, and the proud Pharisee became a lowly, self-sacrificing disciple. – {DA 567.5}
Kindness, one of the fruits of the Spirit, was the manifestation of divinity in Christ that reached his heart. See how Romans and Titus connect this for us:
"Or despisest thou the riches of his goodness and forbearance and longsuffering; not knowing that the goodness of God leadeth thee to repentance?" (Romans 2:4)
"But after that the kindness and love of God our Saviour toward man appeared, Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; Which he shed on us abundantly through Jesus Christ our Saviour; That being justified by his grace, we should be made heirs according to the hope of eternal life" (Titus 3:4-7).
And we see another soul who was truly cares netted in this beautiful narrative:
"Mary knew not the full significance of her deed of love. She could not answer her accusers. She could not explain why she had chosen that occasion for anointing Jesus. The Holy Spirit had planned for her, and she had obeyed His promptings. Inspiration stoops to give no reason. An unseen presence, it speaks to mind and soul, and moves the heart to action. It is its own justification." {DA 560.4}
So those who are truly converted can be used by God in the most powerful ways--by responding the heart work of beholding the loveliness of Christ in contrast to one's own sinfulness and helplessness. Mary responded to the gospel of God's grace, and her life, like a portrait of God's love in her life bearing fruit, is for this reason a memorial. The greatest memorial ever revealed is seen in her life, and is to reproduced in God's people before Jesus returns, for they are to be like Him!!!