the easiest way to tell is by the colors,the female will have green striping around the face area and the male will have blue and not as fat looking.Depending how big that Jack Dempsey is,its doable for a 55 gallon for now at least.Usually a 55 gallon is good for fishes the max of 8 inches,but people have put male Texas who were 10 inches into a 55 gallon by itself,with at least a female companion that doesn't grow into a monster.People who say not enough space for a 55,only fallow a fish bible,but sometimes its understandable,cause people who don't take the hobby serious,only want a maximum of a 55 gallon to shove 14 inch cichlid in a 55 gallon thinking they are fine for life and want to do less work on the tank.You may get lucky like me who has cichlids that are mostly females and a maybe 1 or 2 males in my 125 gallon,the only male I know of is my Texas,who may or may not make it,due to a injury from a female Green Terror,so you better keep a eye on the female GT when they grow close to 6 inches.They will mess up any fish that they don't like period and its not just lip locking either,they will shred other fishes apart,at least that is what my GT did,who I had to get rid of.