Here is what i found for you (sorry my fronts have not been vented, half are too small and the others i just havent bothered yet since they are still young , the smaller they are the harder to sex unless you witness a spawn ).
First off let me mention that when my very large (12 inch oscars ) spawned i noticed one fish i had thought was the male actually turned out to be the female, its tube was broader or wider and was kind of blunt. The one that turned out to be a male had a tube that appeared more pointed and also it seemed to be at angle pointing back toward the tail compared to the female that seemed to be more straight up and down. Hope that is helpful. I know they are not frontosa but they are cichlids so maybe something can be applied...
Here is a link to some photos of specifically frontosa, i believe it listed size of fish etc.. so that might also be helpful. In photos i have noticed that the male vent area sometimes will be more triangle shaped and the females opening more round or circular in shape when no tube is dropped.
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http://www.cyphos.com/forums/showthread.php?t=17630
Here is another thread that showed all closeup pics of females, the person wanted to confirm that none of his fish were males. Some of the best closeups i have seen, lots of detail of a female frontosa vent. just click on the link in the first post then click on each pic to enllarge or use the next button.
http://picasaweb.google.com/dezman69/VentingFrontosa#5275080073708816402
Dont feel bad if you have no or only limeted success in venting, its difficult even for experienced people to do. I have trouble myself and am not particularly good at it. LOL