female will look like oo instead of < o there are some threads on it here if you search venting or sexing .
i was in the same boat as you a while ago . i found taking a pic like you did and then looking at it on the comp was easier then trying to tell when looking at the fish its self .because i was able to blow up pic and see better .
edit sorry i see you did read the venting thread oh well anyway might be handy when you do look for one to have your camera ready to snap a pic then blow it up for a better look