convicts will sex themselves. if they feel comfortable and you have both sexes - they will breed - end of story. They are somewhat sexually dimorphic, but many a female will have impressive filaments on her dorsal and ventral fins. Females tend to be a bit more colorful - but given the ages that convicts have been bred in farms around the world - and considering geographic variants if you are lucky enough to get some wild ones or F1 types, it's a crap shoot. As with all cichlids that aren't all that sexually dimorphic, it's always a good idea to get about 6 and let the law of averages get you a member of each sex.
I suggest just watching them and letting nature take its course. Feed 'em, change 1/4-1/2 the water every week, throw in a clay flowerpot or two on their sides, see what happens.