Until you see which one actually lays the eggs, keep in mind that there are many species where the female is the one to instigate spawning.
My previous post was based on my recollections of my Silver dollars spawning about 1 month ago- I just watched my silver dollars spawning this morning. The females are definiately the red hooked ones, the male is smaller w/ two black spots behind the gill plate.
I watched as the female repeatedly "shot" out 7-10 eggs at once while the male wrapped his anal fin around her. I'm still trying to get a decent photo/video of the female in the act, but at this time I do have some pics of the vent (quite swollen).