I have a pic of a male and a female dempsey that I found on image search.
The male has alot more blue spots from head to tail with a longer snout that kind of looks like two humps from his nose to his forehead.
The female has less blue spots on the back half of body with a short slope from nose to forehead. Also she will be smaller at about 4-5 inches in length..the males will reach 6-8 inches in the same growth period.
You should have good luck finding a young male to grow with your female to eventually breed. If you get a grown male you run the risk of them not getting along after they start courting. But it is worth a try..if one rejects the other you can always get a different male. Be prepared with another tank if they do breed because the male can go nuts and intimidate the female to the point of ignoring the nest, in this case the male needs to be put in the other tank. Then the female will regain her composure and get back to the nest.
The small pic on left is a male, the big pic to the right is what a female usually looks like.
well based on those pictures i would lean more towards male since mine already has more color than that large female.my dempsey does have slight blue splotches on the gill plate, but i have been told that this method is only 100% accurate in wild dempseys.i am not sure of your level of expertise nor am i questioning it but it seems i can always use another opinion.
so far nobody could tell for 100% sure, maybe its just still to young to tell?