What is the sound Islamic course of action if someone close to me told me he knows something bad about my wife that he saw her doing before our marriage, and I want to verify what he said?

The person who informed you about your wife's actions before marriage committed two forbidden acts: The first is backbiting (gheebah), which is mentioning a person with what they dislike, and Ibn Hajar al-Haytami considered it a major sin. The second is exposing the privacy of a Muslim woman, whereas he was obliged to conceal her, based on the Prophet's (peace be upon him) saying: "Whoever conceals a Muslim, Allah will conceal him in this world and the Hereafter."

As for you, you must think well of your wife and turn away from those who slander her, based on Allah's (the Most High) saying: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." And do not delve into her past, based on the Prophet's (peace be upon him) saying: "Do not spy and do not eavesdrop," and his saying: "Do not backbite Muslims, and do not seek out their faults, for whoever seeks out their faults, Allah will seek out his faults, and whoever Allah seeks out his faults, He will expose him in his own home." So, cast aside the whispers of Satan.