How do you know ender is intelligent?

I always found it odd that any six year old could do what Ender Does. Most six year olds I know still have to remember to use the toilet. Still, here is a quote that illustrates Ender's intelligence:

"The teacher would know of course, that he wasn't paying attention, but she would'nt bother him. He always new the answer even when she thought he wasn't paying attention." (pg 2)