As mentioned before, you shouldn't be able to feel the Spiritual Sense of someone with cultivation much higher than yours. Of course, there were treasures and other methods that were capable of achieving such a thing. However, Phex and Darian could tell that Rean and Roan didn't have anything like that in their bodies.
Rean shrugged his shoulders before explaining, "This is an advantage only someone born with Light Element Affinity would have. Spiritual Sense has Life Force, so when it touches me, I can feel it. The stronger it is, the more clear it becomes. Senior Phex and those other Stage Nine Demon Beasts' Spiritual Sense look like enormous suns in front of my eyes." Sure enough, Rean didn't really explain how it worked. However, it wasn't completely a lie either since it was an advantage of his Light Element.