How did Denny know that Enzo the child was actually his dog reincarnated?

The understanding that the child was Enzo reincarnated is primarily rooted in his ability to speak about racing attributes that Denny had previously taught Enzo. This connection is deeply significant because it illustrates the bond that Denny and Enzo shared throughout the story. Enzo, as a dog, had an intrinsic understanding of Denny's passion for racing and had witnessed pivotal moments in Denny's life.

When the child demonstrates knowledge of these racing attributes, it suggests a continuity of the spirit and consciousness between Enzo and the child. This familiarity with racing not only validates Denny's belief in reincarnation but also emphasizes the themes of love and connection that persist beyond physical forms. In this way, the child's insights become pivotal evidence for Denny's realization that Enzo has returned in a new form, enriching the narrative with deeper emotional layers surrounding loss and renewal.