Stare at the squared one at the bottom right and thing about it as face down. Then you can move to the others and see them face down, but you may need several tries.
It all looked like pills to me until I read the text. Couldn't even see plates for a while.
Anyway, I noticed that the top right rectangular one doesn't match the perspective when seen as face down. All the other ones are round, so they don't insinuate a perspective at all. That's why that plate is the key.
Shadow says its face up though. There should be a thicker bright line down the side if it was flipped over. The shadow should not have the same depth as all the other dishes.
They all started as face up to me as well. The one that stood out to me though was the rectangular plate in the top right. That plate looked face down to me which then triggered all the plates to look face down.
I have tried some of the "tricks" in the comments and none work for me. Try as I might, I can only see them as face up and nothing else. It doesn't make sense to me that they could ever appear face down because the lighting wouldn't make sense.
I got to see them upside down for a moment and it does not look correct. They are actually face up in the picture but it is possible to trick yourself into seeing them as upside down but they don't look like they would if they really were
Hypothesis: they are really all face up, but if you read about them being face down before paying attention to the picture your brain turns them face down until one of them triggers the correct position.
I think it's the shadow cast onto the largest round plate by the lower divided round plate
Without that shadow, the plates could be face-down and illuminated from the left, but the shadow would only exist if the light source is on the right, so it snaps everything to conform to that
For me it was a bottom small one that snapped them to right-way-up. With practice and effort I can flip them upside down again, and confirm that I don't struggle seeing the big one as upside down, but still struggle with some of the bottom ones.