Luck and Coincidence
'As I thought, they have taken action at the very first incident.' The Coincidental Luck System thought, using the senses of Rhizen to look around. It then gazed at the three people beside, failing to receive any Mental Energy streams from them, 'This is by far the easiest way to determine if someone is a System Slayer/System Host or not.'
Considering the method Compass Carburettor used to shield his Mental Energy form, it was wise to think that others too would do it. After all, this was the only way to prevent their thoughts from being known to their foes.
A System Slayer was only capable of determining the emotion behind the Mental Energy stream. But, a System was able to understand all the thoughts within the Mental Energy stream.