A Christian, first and foremost, is a sinner, and secondly, he is one who recognizes it, and thus, asks for forgiveness in order to absolve it.
...and, I don't need to add, that the recognition and request for expiation, is based on the acknowledgement of the existence of a holy God, and his offer of grace.
This conviction is what separates us from all other religions (yes, Christianity is a religion i.e. a belief system).
Can one identify a Christian from the rest of the world, even if they were to overtly profess it, yes and know. For a real Christian can be either or both, a conqueror and a defeatist, as much as an insincere Christian can be either, successful in many ways, giving the impression that God is on their side, or they too can appear unimpressive and average.
You just can't really tell as a rule. Some do stand out by their character, where if I had to guess, I would've said that they're definitely Christians. And others, where I would've sworn that they weren't, turned out to be bible-thumping, holy-rollers.
Ideally, if Christians would live according to the promises and exhortations in the Bible, their identification would be much more obvious. But unfortunately, very few of us have truly conquered the world, and loved God and each other in the manner that we are supposed to, and thus, our convictions and status with God, are somewhat often concealed.
For again, a Christian is an incorrigible sinner seeking grace, mercy and absolution from the supreme, holy and gracious God.