Blackcat brings up a good point about looking at how/how quickly your child picks up concepts or figures things out. We are noticing this with YDS5. All it took was reading him one story about silent letters and he immediately started listing words that have silent letters and pointing them out in stories (words he hadn't seen in that way before). Meanwhile, in the third month of K, they are still learning letter sounds. He was curious about an acronym yesterday and I explained what they were. Now he spots them and calls them out. Etc.
We also have had an "enriched" environment, so that, to me, explains things like their extensive vocabularies (the younger one used substantial correctly yesterday). It doesn't explain how quickly they pick up math concepts or their frighteningly accurate memories.
