I always told students at the coop that you should be able to see a full "ring" of the bearing cage with it seated in the cup.

Like this-



If the bearing were flipped, the ring would be broken up, and there wouldn't be that full, continuous ring of the bearing cage.

** No idea why that picture has a scalpel to get a caged bearing out of a cup, it was the first useful image from a GIS of "caged headset bearing". **

EDIT: that's a damn good FBNY post.