Best method i've found is the overall shape of the con and the red on the belly. To breed cons you don't have to do anything out of the ordinary. They are the "Just add water Fish"--LOL...I have about 100 or so fry right now from a black female and a pink male. I keep my temp at about 80 degrees F and change 25% water once a week. You'll find that both parents are awesome parents attacking anything they feel will endanger their young...When you do have fry--remove them when the female lays another batch of eggs. This prevents the parents attacking them out of fear of the new batches safety.
GoodLuck,
Chris