How do you know if a coworker is stupid?

Start off the conversation with "my IQ is 30, what's yours?"

Someone who appears "stupid" at first can surprise you. No matter how brilliant and superior you are, there will be times you need someone to cover for you, to have your back. Someone who was muscled by a difficult, bullying question may not be up for that.  

When they keep asking you to do something for them and you made very clear several times that it will not happen and they just won't take no for an answer because they keep thinking that when they ask again that will be the time you final cave into their selfish demand.

By checking out your coworker's car for a Trump bumper sticker or a MAGA hat in the back window. If you find such evidence, it's proof your coworker's stupid. Another way is to ask, "Did you know that masturbation causes hair to grow on the palms of your hands?" If your coworker then looks turns his or her palms up and looks at them, your coworker is stupid... and a masturbator, but who isn't?

Ask them where they see themselves in 5 minutes time!  If they think about it, you have your answer.

Just count the number of times you roll your eyes while at work. If you can't see right find a different job asap.