Top: pollinated. Bottom, not.
You can tell by color and how plump the top one is behind the flower. Also, unpollinated flowers will fall off. I’m not a botanist so I can’t tell you what the parts of the flower are called.
I read that this cactus was not self-fertile but brushing the flowers together has produced fruit. It doesn’t mean the seed is viable but I will try down the line.
One of these cacti... is a slaaaag.