thats what i always thought was blotches in the dorsal fin but im not sure if thats 100%, hopefully someone more experienced with these guys will chime in
On very very rare occasions, you may find a male with a black blotch on it's dorsal fin. Skip from real hard cichlids has such a male. So it's not 100% but maybe 99% lol.
i don't know if thats completely true, i ordered two males from rapps (asked for them) and they both have big black blotch. I'm hoping he wasn't wrong on sexing.