Visually determining gender among yellow labs [excluding venting] can be challenging (and near impossible with juvies).
The presence or boldness of black fin markings may offer clues but aren't a definitive indicator, since a high quality female can have 'better' fin markings than a lower quality male (or a mature male can have 'better' markings than an immature male).
How the fish behaves, especially when among group of other yellow labs, contributes to determining gender. Males may vie with each other in attempts to establish dominance and initiate efforts to attract and spawn with the females. While it probably happens, I've yet to witness two confirmed female yellow labs sparring with each other.
To a lesser extent, the yellow lab's interaction with other mbuna species may offer additional clues (do aggressive males of other species view the lab in question as a rival or a subordinate tank mate, or spawning partner, etc). When some of my male metriaclima and pseudotropheus species decide to 'clear the immediate area' of other mbuna in the vicinity, I've noticed that a half-heartedly 'charge' seems to easily ward off female yellow labs, but the flaring of fins and other threatening behavior may be required to motivate a male yellow lab to move. Dominant male yellow will sometimes 'square off' with other male mbuna's. (I'm convinced that the fish themselves 'know' which fish are male and female in the tank).
Observation of physical appearance and behavior should give you and idea as to the gender of the lab. Interpreting these variables seem to work best when the fish are already in the tank, than at the LFS while trying to handpick particular genders from what ever tank they're in.