how does jonas know he might be able to see beyond?

Jonas knows something strange is up. He can perceive things other people can't. Objects change in front of him and besides, he looks different. Jonas has pale eyes while the rest of the community conforms to a sort of genetic freak show (they all have dark eyes). After years of stalking Jonas the Elders make him "Receiver of Memories". This means the community thinks he is forever creepy and Jonas gets to spend the next few years locked up with some old guy. The old guy, the Giver, gives Jonas a massive memory injection. Jonas is not only able to see the past; he sees shades of the future.

Jonas realizes that he was different when started to see things differently, Like the apple mentioned in the book. Also, he notices that his eyes are pale unlike everyone else's.