Well, your not getting any responses because, it is never too clear. IMO, looking at the belly of a fish is a very poor angle to observe the genitals of a fish.
In the past, when fish forums were frequented by many more posters, venting threads often had an equal number of posters saying the fish is female and male
About as useful as a coin flip.
I've never given an opinion on the sex of a fish from a belly shot, as the so called 'venting' is not my method, at all. It is definitely not foolproof as I have come across thread after thread where fish that were vented end up proving to be the other sex (especially Malawi that were vented male end up holding eggs). I suppose it is possible for someone to become expert enough at the method, for it to become reliable, but IMO, a one time method doesn't take into account the degree the area varies over time.
This my opinion, and i have lot's to say about it. I've got an example that demonstrates it's flaw. I've put my face to the tank, and observed small demasoni do the breeding act up close. Like a lot of mbuna, there are no breeding tubes protruding (yes sometime female mbuna have some breeding tube protruding and I speculate that it relates to the number of eggs; males really don't). But while there is no breeding tube protruding, the area that the egg is coming out is proportionately HUGE compared to the size of the fish. If a female that is close to laying eggs is compared to a female that is not about to lay, the latter could easily be identified as male, based on the size of the area. The method doesn't take into account that the size of the area is in flux, according to how close the fish is too breeding.
I observe the breeding tubes over the coarse of weeks to months, while the fish are in the tank, generally from a side angle, but from what ever angles I can view it from. Females go from nothing at all protruding, to very large , thick and blunt, shortly before laying. Females tends to change a lot over time. Mature males will often have a small pointed tube protruding most of the time. It will get longer shortly before fertilizing eggs, but is always pointed at the tip and is usually skinnier then the female's. On a large fish like oscars, this is pretty easy to observe, but you have to have some patience as it needs to be observed over a period of time to confirm. Malawi mbuna are sort of an exception to this method, as they seldom have enough breeding tube protruding to tell much. But i don't have too much problem with them either, as they are prolific; females hold very soon; my tanks are in the living room and bedroom so i often witness the act, and male's are obvious by their behavior. Now, from pictures taken while the fish is in the tank, it's seldom apparent unless the fish are fairly close to breeding time (excluding mbuna which often don't have much breeding tube protruding).