To the right of the yellow cactus flower you can see how the part that holds the flower looks when the flower is gone. This part will contain the seeds. The fruit (cactus fig) of some species are edible and sold as tuna, or Xoconostle (Tuna agria). I like the tuna a lot and when it's the season we buy them very often. The fruit is very juicy and sweet, and reminds me a bit of watermelon, but has more structure also because of the many small seeds.
I tried Xoconostle once or twice, but the taste was quite different, and I didn't like it as much as the tuna. It might have been that the fruit was not ripe enough.