How did Uncle Axel figure out that Alan knew about the telepath in the chyrsalids?

Uncle Axel suspected that Alan knew about the telepath in the Chrysalids because Alan asked too many probing questions about Sophie’s mutation and Rachel’s baby, which indicated he was aware of David’s telepathic abilities and the secret surrounding them. Uncle Axel's intuition and keen observation skills allowed him to put the pieces together and confront Alan about his knowledge.