What makes the mechanical hound "hone" in on its target in Fahrenheit 451?
If we look at when the mechanical hound is first introduced in the novel, we are given the answer to your question. The mechanical hound can be set to focus on targets based on their amino acids, sulphur, butterfat and alkaline. Note how Beatty presents the mechanical hound when Montag enters work and tells him that he thinks the mechanical hound doesn't like him:
Come off it. It doesn't like or dislike. It just "functions." It's like a lesson in ballistics. It has a trajectory we decide on for it. It follows through. It targets itself, homes itself, and cuts off. It's only copper wire, storage batteries, and electricity.
Thus we can see that the hound hones in on the target it is given by the fire service based on the target's personal levels of amino acids and other chemicals, supposedly producing an efficient and swift way of killing any undesirables. The hound is thus presented as a ruthless and deadly machine that is also safe to the public as it "sniffs out" its victim and despatches it quickly.