Despite people saying the usual about longer fin extensions on males you will find it extremely difficult to sex young meeki.
Males and females can develop fin extensions. Females can show a darker spot to the dorsal but this is not that reliable with meeki.
I advise you get a group and observe their behavior. This will normally seperate the sexes to me before any physical markers.
They do Well in group's and this is how they are found in nature.
As they mature more, males will be bigger and look a little more colourful and impressive. The only way to guarantee a pair of meeki from adolescent stock is to buy them and raise them in numbers.Anything else with adolescents is just guesswork I'm afraid.