Thanks to "Wise Brothers", I have stumbled across some questions to ask yourself. Now if you find that you're guilt of these things, then your coworkers ma find you to be unintelligent.
Do you use emojis in your emails?
Are you guilty of telling inappropriate jokes
Do you swear in general?
Have you found yourself never asking for advice?
If you found yourself guilty of all of the above, your co-workers may think you're not as smart as you are! I've never been one to judge, just watching out for ya ;)