Some have more elven traits than others. For example, while you'll find that most half-elves have pointed ears, there are still a few that have ears indistinguishable from a human's. If they're especially lucky, they could live their whole life pretending to be human.

I suppose this is why Tethe'allans invented a device that can detect elven blood. Though, they only ever use it on criminals to determine the weight of their sentence.