How to tell the difference?
There is no difference.
Aren’t they opposites?
Joseph! I am not gullible (or open minded) enough to believe that statement.
If you are very open minded,all your brains fall out.
How are open-minded and gullible opposites?
Gullible means believing whatever people tell you, or not being able to think for yourself. I think that’s exactly the opposite of open-mindedness.
The opposite of believing everything that people tell you is not believing anything that people tell you. The true difference between gullibility and open mindedness is that, as you say, a gullible person believes everything anybody tells them. An open minded person is prepared to believe what anyone tells him but needs to evaluate every statement himself.
If a brain is too closed it’ll suffocate.
And what Redleg said.