It depends on the species. Green, turquoise, and rotkiel males usually have a worming pattern on the cheeks, gill plates, and across the bridge of the nose while females have a mostly solid face.
Mouthbrooder males often have small freckles in the face and gills and more red, but it's subtle and I've seen dominant females with just as much color and the same spotting. It's not always reliable.
Notatus males have black spots on the face and gills while females do not.
Golds and super reds are tricky. Because they are line bred, females will sometimes show facial spotting like the males. In most instances with golds, males develop the red lines/spots in the face and body while females are more solid, but this isn't always the case. Super reds all have spots on the face.