What caused Rainsford to believe Zaroff knew he was hiding in the tree?
I assume that you are talking about what happens very early in the hunt for Rainsford. This is the part where Rainsford has first run away and tried to leave a confusing trail. Then he has climbed up in a tree to hide in hopes that the trail will confuse Zaroff and the dogs.
What makes Rainsford think that Zaroff knew where he was is the way that Zaroff acted. He looked in the very tree where Rainsford was. His eyes went almost to the very sport where Rainsford was. Then he smiled and blew a smoke ring. It seemed very much as if Zaroff knew where Rainsford was. Here is a quote:
The general's eyes had left the ground and were traveling inch by inch up the tree. Rainsford froze there, every muscle tensed for a spring. But the sharp eyes of the hunter stopped before they reached the limb where Rainsford lay; a smile spread over his brown face. Very deliberately he blew a smoke ring into the air; then he turned his back on the tree and walked carelessly away