Detect Evil is a tool just like anything else. That means it should have a defense.
Since I tend to analogize everything to war, I will do that here as well. There are two forms of war, maneuver and attrition.
Maneuver attempts to attack and enemy when his defenses are down, unprepared, or mis-prepared. So in this case, a divination should be successful when someone doesn’t expect it or expects something else. Perhaps I head fake that I will Detect Lies and instead Detect Evil. In 4e this is because I bluffed and you failed your insight check. It might be because I snuck into a place where you didn’t know you were being observed, or mistook the observer for something benign , a waitress, for example. One might analogize to eavesdropping. If I can catch you unaware so that I can listen to your secret conversation, I can Detect Evil on you with your defenses down. You may still have passive defenses, but active defenses are down.
Attrition is where two sides spend or bid against one another both sides hoping to deplete the resources of the other side until at some point one side runs out and basically collapses. If I suspect I have more magical resources, I might spend them to first reduce your active defenses and then penetrate your passive defenses. But this will only work if I believe that I have more resources, otherwise I’ll spend them all and you’ll still have defenses.
Now in 4e, this kind of thing can be an at-will power, a per encounter power, daily, and a ritual. Like all the other things that are no longer tracked to eliminate bookkeeping, this system is easy to use, but it reduces attrition to “who gets a luck roll first”, though maneuver is still possible.
Something like this could be an encounter, presumably a Wis vs Will attack. Success would reveal information.
However, Discern Lies is a ritual, and so unless there is a good setting reason to do otherwise, the halfling ability would be a ritual as well. But that might be the case, since the halfling ability is more of an instinct or aspect of perception than it is an actual divination, like the spell was. It was an innate ability, not a consultation with the heavens. Even so, if a halfling has to meditate, concentrate, or get himself attuned, it can still be a rital.
As others have mentioned, the halfling is really just sensitive to shadow world things. Not precisely evil, but the taint of the Shadow World. This is not the only example of this particular power. Though they were spells, we had Determine Cerilian Origin and Detect Elven Influence. All three of these seem to be of a piece, and should be treated the same way.
A kind of ritual or ability to detect Shadow World taint, Sidhelien influence, or Cerilian origin does seem to be of a similar power, even if the power is expressed differently.