Sorry but generally that doesn't work, maybe in some closely related strains but unlikely electric yellows are monomorphic or whichever it's called
The only way to sex them for sure is to vent them pic em out of the watter and check the holes under the anus, females have bigger holes.
Behaviour can give useful clues dominant members are usually males, males more likely to shakey, females seem to show their throat a little more often when fighting,-stuff like that, but you can't be sure unless you vent. Luckily Labidochromis caeruleus are able a fairly mild hardy species and you are usually safe from agresion and breeding problems without haveing to ensure a male female ratio
