We've had to have our older, gifted child stop calling his younger brother "my little butthead" after the brother got a 99.1% on the Raven IQ test and > 160 on the RIAS. I actually wasn't sure he was gifted since for a long time, he would say things like "I do-ed it," instead of "I did it." Etc.
Sometimes you can't tell! He's reading at a third grade level in first grade now and I think he is just taking off. Our older boy has always been super articulate, using complex words and ideas. Our younger one never did- he watched alot of Sponge Bob. My husband, who seems like a pretty bright guy (he's a lawyer) drinks beer and likens himself to Homer Simpson; he always would say our younger one takes after his (ungifted) side of the family!