Right from the get-go we have something of an epistemological/ontological problem. That is, there are differences that are easier to see, and then there are differences that are more central, but harder to discern. On the ontological side the whole of the order of salvation, or ordo salutis, describes the internal difference. The believer has been regenerated, given the gift of faith, is indwelt by the Spirit, gifted by the Spirit and grows in grace and wisdom. The unbeliever has none of these. Neither the believer nor the unbeliever, however, has magic soul-exposing glasses by which we can judge the invisible changes.
While we cannot see into the souls of others, Jesus tells us we can read fruit. That, however, is not as easy as we might think. Suppose for instance you knew a man who not only was unfaithful to his wife, but was unfaithful to his wife with the wife of another man. When his paramour becomes pregnant with his child he then murders her husband. Or consider another man who spent years studying the Word of God. He boldly went into hostile lands and proclaimed that same Word. He spoke with Jesus in the context of the utmost intimacy. He was even known to cast out demons in Jesus’ name and to heal the sick. It would be hard to imagine two piles of fruit more radically diverse. Yet we would be wrong on both counts if we affirmed the first was not a believer and the second was a believer. For of course the first is King David, the second Judas Iscariot.
Unbelievers are more than capable of living visibly righteous lives. And believers not only still battle against sin, they often lose those battles in public and spectacular ways. Which is why I would suggest that the best distinguishing mark is less that believers sin less, though that certainly may be the case, more that believers repent more. We are the repentant.
I would argue, in fact, that before we begin to talk about the fruits of repentance, or fruit befitting repentance we recognize that repentance is fruit. It is cultivated by the Spirit in us, and blossoms into God-honoring sorrow for our sins, but also God-honoring confidence in His grace. Indeed the fruit befitting repentance, I would suggest, is less the committing of fewer sins, or sins less flamboyant, and is more the fruit of the Spirit. The repentant bear love, for they know they have been forgiven much. The repentant are marked by joy, for they know they have been forgive much. The repentant are at peace, for they know they have been forgiven much. The repentant are patient, for they know they have been forgiven much.
Christians ought to know this. I wish still more, however, that unbelievers would have to confess it. Rather than grumbling that we believers are a holier-than-thou people, I wish they, at least in their moments of honestly, would admit that we are a more-repentant-than-thou people. I pray a day will come when we will be known more for humility than pride, joy more than anger. Until that day comes, however, I will continue, by His grace, as I grow in grace, to repent for my failure fully to obey His law.