It's pretty obvious if you grow out a group, Males double or triple the size of the female and get much larger heads. Venting fish is pretty easy, females have much larger vents than male to accomodate her larger egg laying tube.
I was hoping to see 2 pictures next to each other, so I can clearly know the diff. This is pretty new to me. Also I was searching the site and couldn't come up with 2 pictures of a male and female vent for the red devil.