I mean, maybe you know that they are smarter than you or you are smarter than they are. That's a relative assessment.
How do you know how smart someone is in the absolute sense?
Try to imagine a scale of potential intelligence, ranging from absolutely none to the most brilliant entity you can fathom. I think most people (myself included, obviously) end up closer to the "nothing" end of that spectrum than the "incandescent genius" end.
Sure. We're smarter than rocks. I'm smarter than most of the fish I've met. There's a lot of evidence to suggest that we are significantly smarter than cows.
That doesn't mean we're brilliant. Humans only shine so brightly because of the surrounding darkness. In the presence of a truly massive intellect, I think we would all seem a little silly.
There is a lean principle called perfection, which says you should compete against an imagined ideal of perfection rather than other organizations in the real world. That way, you never run the risk of stagnating when you pull out into first place.
I think we can translate this principle into the humbleness-space. Maybe, as humans go, you're super-intelligent and maybe that makes it hard to be humble. So don't humble yourself before other people. Humble yourself before an imaginary intellect far-greater than your own.