Detection of Evil and/or Good

One thing that seems to come up again and again in AD&D games whenever a paladin is involved is the innate ability to detect evil. Because the paladin is able to use this ability at will, a smart player's intuition is generally to attempt to detect evil around every corner and inside every inn; after all, why wouldn't you want to know if that innkeeper is secretly plotting to murder the whole party while they sleep?

Unfortunately (read: fortunately), neither the spell detect evil nor the ability of the paladin are so keen. From the first edition DMG (p. 60):

DETECTION OF EVIL AND/OR GOOD

It is important to make a distinction between character alignment and some powerful force of evil or good when this detection function is considered. In general, only a know alignment spell will determine the evil or good a character holds within. It must be a great evil or a strong good to be detected. Characters who are very strongly aligned, do not stray from their faith, and who are of relatively high level (at least 8th or higher) might radiate evil or good if they are intent upon appropriate actions. Powerful monsters such as demons, devils, ki-rin and the like will send forth emanations of their evil or good. Aligned undead must radiate evil, for it is this power and negative force which enables them to continue existing. Note that none of these emanations are noticeable without magical detection.

The second edition PH defines the spell detect evil using similar terms. While there's no reason a paladin can't detect evil constantly, only truly extraordinary circumstances would trigger a positive detection. No, the innkeeper probably isn't a demon, and the door to the blacksmith's outhouse probably isn't a portal to the Abyss. Though a player playing a paladin shouldn't feel bad about calling on the ability when a critical situation arises, it also should be accepted that, much more often than not, detect evil won't reveal anything at all. It's easy to trivialize the ability to detect evil when read as part of the paladin's class description - after years of playing D&D, I still seem to do it myself sometimes - but in the end the purpose of the ability/spell is to identify unworldly evil sources, not simply a common criminal or even a malicious mercenary.