You would wonder why any setting with easily achievable spell that unfailingly tells you of a persons alignment and (assuming spells are created and not gifted down from the heavens with a quartet of angels and the spell inscribed in bloody gold inlay) by extension spells that would also unfailingly tell you about a persons likes and dislikes and destroys them if they dislike the original flavoured versions of anything aren't an extremely segregated society cut off from others in fractal subsets with pettier and pettier things being scanned for.
Man that actually sounds like a cool setting.
It's really super falsifiable though. Detect <Alignment> spells are level 1, but so is the bard spell Undetectable Alignment (level 2 for Clerics and Paladins). Undetectable Alignment absolutely foils alignment-detecting divinations, no opposed roll. Under that spell you appear as neutral, IE you don't light up on any of the four detect spells. It lasts 24 hours too.
Though to be fair you have to be a level 2 bard or level 3 cleric to cast the spell, and the spell itself can be detected with Detect Magic (faint abjuration). I think a DC 21 spellcraft check would reveal the name of the spell. With access to level 3 Dispel Magic one could strip away the Undetectable Alignment, confirm it's gone with another Detect Magic, *then* start casting Detect Evil/Good/Law/Chaos until there's finally a (probably) accurate reading. Though I think there are illusion spells which would beat that process as well... I know really epic Bluff checks can. I think it depends a lot on what splat books you use.
And even if you get it right, alignments change. Outsider intervention helps, but adventuring or simple life-changing experiences can do it. So you certainly can't scan a baby and predict anything meaningful... Heck I bet most paladins were chaotic in their teen-equivalent years.
BTW, paladins can't harm innocents. Paladins are lawful good (excluding variants), but they follow some very specific rules waaaay beyond that alignment.
A paladin must be of lawful good alignment and loses all class abilities if she ever willingly commits an evil act.
Additionally, a paladin’s code requires that she respect legitimate authority, act with honor (not lying, not cheating, not using poison, and so forth), help those in need (provided they do not use the help for evil or chaotic ends), and punish those who harm or threaten innocents.
Associates
While she may adventure with characters of any good or neutral alignment, a paladin will never knowingly associate with evil characters, nor will she continue an association with someone who consistently offends her moral code. A paladin may accept only henchmen, followers, or cohorts who are lawful good.
So no, a paladin can't slaughter any innocent orc civilians. On the other hand, a paladin is literally required to hold their party members to their moral code. I think a group that wants the paladin to stick around should probably interpret that *very* loosely, but it's there. No one in the party can "consistently" lie, cheat, *use poison*, cast evil spells (neutral spellcasters are able to wield Good and Evil together), or even refuse to help people. It's simply "Sorry guys, the party has to generally obey my character's morality or I'll literally lose my powers". Some rules are probably best ignored for the sake of party cooperation, is what I'm suggesting.
Edit:
I should probably clarify (because it's fun), Undetectable Alignment makes you show up as neutral/neutral. Like most commoners will, excluding racial or exceptional cultural factors. Basically strong alignment values are what show up on the scan, neutral people just don't emit alignmenton particles. (Clerics and outsiders show up a LOT brighter. Except neutral ones, heh).