So! In a bunch of helpfiles it claims that powerful mages are able to detect latency in people, however this has never been discovered to be the case (as far as I now). I'd suggest a new spell that would allow a mage to detect latency in a person, either as a passive effect when you look at someone. Or perhaps having cast it on a singular target, the drawbacks would be this would be an obvious spell so best not done in public to sweep the taverns for latents etc. To make it still viable if it is an obvious spell, it could be cast at a distance on a familiar/remembered target.
I'm not sure what element it could sit under, but perhaps void due to the ephemeral nature, or fire.
