Well if they are sexually mature, then a sure fire 100% way to tell is the net them and feel the lower jaw. The females lower jaw is smooth, but males will feel rough and bumpy. Males grow tubercles under their jaw that they rub on the female to get her to release eggs. There's other ways too but anyone on this site can tell you that using fin and body shape is never totally accurate.
Sent from my iPhone using MonsterAquariaNetwork app