The basic Vocator (imperial Evoker hunter) cannot sense Relics or other Evokers outright through any magical sense. But they are trained to recognize the subtle signs present in most Evokers. And the not-so-subtle signs too. There may be one or two in the imperial army capable of detecting evokers, but he or she (or they) have yet to be unveiled. Mind you, these Vocators primary goal is capture, not to kill. The Empire doesn't hate Evokers the way the Idris do.

While the Kalesians generally believe that evokers should all be controlled, the primary goal at this stage of the invasion isn't to capture Evokers. Sure, they won't mind if they do capture some, but its far easier to catch them after they have full control.