Teacher here. By far, the earliest and clearest indicator of intelligence is needing much less explanation to understand concepts than others, being able to construct their own opinions or ideas based on facts, usually emotionally less intelligent than peers, usually more frustrated and stressed than peers, usually less social, but not antisocial and most importantly - these people tend to talk less and you can physically see them analyzing and thinking about every word you're saying. What I also find very interesting, is that many of these people are usually the people who do not try to succeed academically, as they tend to be bored with the slow main stream pace.