It surely cannot be denied that when a person is intelligent, they become attractive regardless of race, gender and age.
It shows from a conversation. People can tell too if you're just acting smart or you're really smart. You don't have to talk about science, medical or any of that sort to show but what comes from your mouth in a conversation is sufficient.
Crappy things that you say will not impress people. Not everyone has the intelligence and develop it. If all you ever have is crappy stuff in your head filled with lame lines, then you may find other things to be attractive such as being humorous.
Personally, I feel intelligence beats all and comes in second is being funny. That person automatically goes to a higher level when he or she is intelligent. You look at them differently. Of course, that includes not being narrow minded, that is related to intelligence too. :)