When you plan an elaborate treasure hunt for your DD's sixth birthday and make a point of using rhyming clues with what you think are simple words, only to discover that only your DD and her BFF can read them.

When your seven year old has read the Harry Potter series so many times that she has large chunks of it memorized.

When you take the enrichment homework the gifted coordinator has selected for your older DD and give it to her younger sister, who zips through it and asks if you have anything harder.