With my Australoheros, the differences were pretty obvious when together, and they were old enough
Males were longer and had less melanin, in the dorsal, and overall body.
Below a male, in various mood coloring.

Note the slightly raised profile, below when a little older,

now below some shots of females.

the head shape a bit more streamlined and she is about 1/3 smaller at the same age

also note the dark area mid dorsal on the female, males have much less melanin in the dorsal fin.
Now in spawning mode, they are both almost indistinguishable in color, just not in size.

Even from above, in the pond, the females dark area in the dorsal on the left of the pic stands out.
In most of the pics above, the male is about 5"-6"+, if yours are young, not quite mature, and small, its not so easy to tell.