How do I know if I am one of those whose good deeds will be rendered worthless because they violated the sacred limits of Allah, and how do I protect myself so that I do not become one of them?

The aforementioned Hadith describes people who will come on the Day of Resurrection with great good deeds, but Allah will render them as scattered dust because when they were alone with Allah's prohibitions, they violated them. These individuals outwardly display righteousness to people but commit sins in private. Such an act is more harmful and misleading to Muslims. And whoever is in such a state, either he is a polytheist if he believes that Allah does not see him, or he is disdainful of Allah if he knows that Allah sees him yet still dares to violate His prohibitions. As for someone who performs acts of obedience sincerely but is occasionally overcome by his lower self in private, and conceals his sin, we hope that he will not fall under this warning.

Prevention is achieved by seeking Allah's help, observing obligatory and supererogatory prayers, diligently performing private acts of worship, reciting and pondering the Quran, being wary of persisting in sins, and promptly repenting.