(edited) I was just checking on that myself for my Firemouth. From the numerous articles I read, sexing by the vent refers to the papilla, which is near the vent where the eggs or sperm exits. The female has a larger, rounded one and the male has a smaller pointed one. During breeding time is when you are likely to see it, located between the anal and pectoral fins. It is a small, nipple shaped projection. First I heard of it, since up until recently, I have kept livebearers.
Addendum...
I've been practically standing on my head checking out my Firemouth. I very nearly needed a magnifying glass, but I was able to see what I've been reading about. Slightly bigger than the size of this period "." and yes, pointed (not quite comma-shaped, but similar), but then, my fish is only about 3" or so. At first, I thought it was the start of the anal fin, but it is distinctly separate. My large convict has the same distinction, but he was much easier to sex because of the length of fins and the bump on his head.
Here is a pretty good article with photos.
http://www.fishhead.com/articles/ventsex.htm
Addendum 2:
After looking at the photos in the above article, I thought I could not have possibly seen what I was looking for, but I remembered that I had picked up a pair of jewelers glasses at a garage sale and I was able to confirm that the Firemouth is indeed a male. It also helps to have a contrasting background with light between the fish and the background.
