Research and data. A Wizard can trust nothing less, especially when there's too much world for one to observe entirely on their own. We've got standardized measures for things like capacity for complex learning versus empathy and force of personality. Dogs tend to be a fair bit better than humans in empathy on average, but their ability to learn is extremely limited compared to the same baseline.

Of course, there are exceptions - Awakening can make them as smart as, or smarter than humans. Animals who become wizard's familiars immediately gain quite a bit of intellect, and moreso as the wizard gains more power.