Sincerity is visible not only in worship but also in ordinary dealings: keeping promises, returning trusts, resisting envy, and apologizing for mistakes. The full state of a heart is known to Allah. Self-examination is safer than issuing certain judgments about another person’s hidden motives.
An effective sequence is to set a sound purpose before a task, avoid injustice while performing it, and resist pride after succeeding.
أَعْمَالِكُمْ is the plural of عَمَل, action. Its plural form suits the wide variety of small and large deeds through which character is displayed.