Quote Originally Posted by LineageRazor View Post
How do you tell? Unless I'm misremembering, nowhere in the story has anyone come up with a device that you can scan someone with and say, "Yup, they're tempered."
Unfortunately, at least one Scion could have detected the tempered, namely the aethersight-gifted Y'shtola. This was what she said in the aftermath of the qalyana's summoning of Lakshmi at the peace talks:

Quote Originally Posted by Y'shtola
A primal's servant is betrayed by his aura. If you mean to conduct an investigation, I can identify any others acting under Sri Lakshmi's influence.
To be sure, Raubahn himself conceded that they "were all caught unawares", adding that they'll take the incident "as a lesson, and watch our own more carefully in future". He was specifically referring to the Alliance's own soldiers who helped smuggle the crystals into the throne room. It really does appear that the Alliance were caught with their pants down this one time, as it did not occur to any of them to check their own ranks for tempered individuals.

The sobering lesson, it seems, is that the tempered could in fact disguise their mental state well enough to pass for ordinary. That may seem like an obvious mistake in hindsight, but it has to be seen in, I believe, in the context of the Alliance's experience with Eorzean primals: All the tempered so far usually display clearly warped behaviour, due to the nature of their tempering by their respective primals.

Lakshmi was different, as others had pointed out. Her summoning was based on the desire to forget grief by losing oneself in an illusion of bliss, and not so much in a desire to protect or expand a tribe's influence. The nature of her tempering was likely affected as a result.