Detect Evil is both a spell and a paladin's "spell-like ability". I'll focus on the latter here.
- There's no in-engine or in-game actual implementation of the ability, so everything has to be handled with IC emotes and OOC tells.
- Players cannot demand that other players comply. Only a DM can do that.
- Spell-like abilities have no "components", a character only needs to concentrate to activate one. They can't be counterspelled, but do provoke attacks of opportunity.
- Detecting Evil has three phases, each taking roughly six seconds.
- The first round of concentration reveals whether there are evil auras present at all, in a 90° arc and within 60'. So keeping your distance or standing behind the paladin keeps you "off the radar".
- This round reveals the number or auras present, as well as the power of the strongest one.
- The exact strength and location of each aura, or strength and direction if there is no line of sight.
- Detect Evil only scans for evil auras. It does not detect traps, poisons or evil thoughts, unless they have an actual evil aura. Things with evil auras are:
- Undead
- clerics of an evil deity (regardless whether they are evil themselves).
- evil Outsiders (I'm not sure if that means Outsiders with the [evil] subtype, or just any old Outsider who happens to be evil. If that's the case, Planetouched PCs would qualify.)
- evil items
- spells with the [evil] descriptor
- evil creatures (again, I'm not sure if that requires the [evil] subtype, or just an evil alignment. Ask your DM.)
- Detect Evil is blocked by:
- 1 foot of stone
- 1 inch of metal
- a thin sheet of lead
- 3 feet of wood or dirt
- the Undetectable Alignment spell (bard 1, cleric 2, paladin 2(!) )
- As always, the SRD has the exact details.