We did a WPPSI-IV test when DS was in his mid 3s. We thought he was bright. There were a few earlier clues (many from the lists of "characteristics of gifted kids" you can find out on the web).

We tested more to rule giftedness out, but the results came back as gifted. It was pretty shocking because he seems pretty average in a lot of ways. It's still early days, and IQ can move around, but the gifted school he is in seems like a very good fit.