You can't see into another person's heart to detect his motives. I've been accused of, and punished for, perceived showing off by people I never tried to impress in my life, just for living and speaking and writing the way that came natural to me. Some people can't stand the idea of normal individual variation and will scorn absolutely anything, from disease to genius, as fake if it falls outside their perceived normal. Some people are obsessed with what other people think about them, and can't conceive that this isn't the number one priority for everybody, but aren't honest enough to admit it. Some people want to make themselves feel special by pointing out how other people are trying to be special.
If you must judge people, do it by their actions. Do they say something and stop and wait for you to be impressed? Do they only ever talk about the thing that supposedly makes them special, dragging it into conversations willy-nilly? Do they move quickly to one-up someone else with a similar trait, denigrating it or making superior claims? Then yeah, they're probably seeking attention.
You know what? They probably need it, too. If people would accept who they were, and love that person, they wouldn't need to pile on the outre behavior just to be noticed. A sense of humor, an open, accepting attitude, and polite honesty ("Really? You saw that in a dream? Did you write it down? Oh, next time you have a dream like that write it down, then you can prove it to skeptics.") will do them, and you, more good than writing them off as posers and trying to make them feel even worse about themselves.