This particular trick is a kind of slug that she puts into your head through your ear. Once it's in, it wraps itself around your brain, taps into your thoughts, emotions and memories, and controls your body.
They tend to react to things she herself would react to, like if you got someone from Lacerta or Hydra to come talk to you it'd probably give you away. Scanning the brain for irregularities also works - Bones on Canis can do that.
Once it's in the only way I know of to get it out is to have Lepus cut it out manually.