As for me personally, I have to interview a crapton of people. The standard logic questions "4 people crossing a bridge", etc. After a while it becomes natural and you can just sort of tell. Granted, I can't really tell with anyone below 120ish because those people never pass the case part of the interview and don't get a verbal one. It's different when you verbally quiz someone with questions designed not to differentiate those with 100 and 130 IQs, but those with 130 and 150+.