The smartest folks are able to admit when they aren't familiar with a particular concept. As Jim Winer writes, intelligent people "are not afraid to say: 'I don't know.' If they don't know it, they can learn it."
Winer's observation is backed up by a classic study by Justin Kruger and David Dunning, published in the Journal of Personality and Social Psychology, which found that the less intelligent you are, the more you overestimate your cognitive abilities.
In one experiment, for example, students who'd scored in the lowest quartile on a test adapted from the LSAT overestimated the number of questions they'd gotten right by nearly 50%. Meanwhile, those who'd scored in the top quartile slightly underestimated how many questions they'd gotten right.