I can tell that I got the focus right on an animal portrait when I can see the texture detail in the iris. This works because, if the animal is looking directly at you, the iris is on a two-dimensional parallel with the sensor in the camera. If it’s perfect, you can see the detail all around both irises (assuming a two-eyed animal).
It’s surprising how many photos out there wind up being out of focus when you know what to look for.
It’s also surprising which animals have more and less iris three-dimensionality. (And which ones don’t really have eyes that work that way at all.)