What does the speak of " zen Jardin De Los Espejos Quebrados, Caliban Catches a glimpse of His Reflection" mean when he says, "To call a man a beast, one must see within his heart"?

He simply means the ability to see the true nature of an individual beyond his projections ( which is a facade ).

Explanation:

Most times, most people has a dual personality of their image which they project to the society. The have their true self which is mostly hidden and their mirror self which they society sees.

Most of the worst people who have ever committied crimes against the humanity like Hitler, Stalin and the rest always projects the image of goodness to the society in which they live in while growing up. Majority never knew that they were beast in human form that was completely masked. It was only when they were opportuned to assume power were they able to unleash their real nature.

In summary, the ability of people to see the heart of a man in his actions towards little things would show how noble or beast his innermost heart is.