<You are correct. There is no outward sign of a yeerk-infested being. They behave the same, speak the same, and carry on what would be considered a normal life for their host. They hide themselves in a population, slowly infiltrating more and more beings, until they are able to take control by force. Your planet is plentiful. There are many, many people who would revolt, but that is also why they want your planet to such a degree. Your people are unaware of the growing threat that endangers your freedom.>
"So what do we do? We're not gonna just start chopping heads and looking for Yeerks. Could we do like a medical scan or something?"
The bird laughed in my mind. <Hahahaha.... Your kind does not have any technology advanced enough that could detect a Yeerk.> Yeah, the alien-parrot had an ego, alright. <There is one way to identify a Controller. On their homeworld, Yeerks swim in pools that cover most of their planet. They absorb nutrients from the sludge, mainly particles from their sun. These are called 'Kadrona particles' and the Yeerks are dependent on them for survival.>
"Don't tell me that they feed off of sunlight..." I started to say.
<No. Your sun is far too young to be producing noticeable amounts of Kadrona particles. Every three days, a yeerk must leave its host and feed in specially constructed pools that are filled with artificially-manufactured particles. That is how one can tell. After three days without feeding, the Yeerk will die.>