i wonder

[ausarow]

when you have a mature enough male in breeding condition, with all the right cues, you can knock em out, roll them over and give their belly a squeeze ( down towards vent), if you get milt its a boy. if your really good and have seen a lot of male and females and know thier sex you get to know what a male vent looks like compared to a female vent. once again best done in mating season/ conditions.
if no milt is present its either a girl or he aint in condition. but thats the best way in that it sometimes will definately show you a male.. other than that boys are often longer, thinner, not as tall from pec fins to dorsal, the mouth often juts out a little more overbite, the males can tend to dominate at feeding time in breeding season.
then you could always run a female in condition under an ultrasound scanner.

jardinis- it is said the female broods the eggs and not the male, so it may be different for their mouth shapes. funny if that holds true everywhere though becasue one point in time they went from female to male holding the brood in terms of evolution.

males may also have longer pec fins than female and the main bone holding the finnage can tend to be a tad thicker than a female. its rather rare for a female to spit out eggs without a male around but it can happen, but then if that was the case you would have all the spawning cues happeneing too and i dont think many even know or have spent any time in trying to work them out. which is why no one can breed them outside of their natural ranges..
so any of these tricks are not gonna be any good to anyone really apart from the observations and the standard length tests. fish can always deviate out of the normal so thats only a ready reference guide.

 

[ausarow]

eggspots by the tail?? do you mean a gravid spot like on a guppy?? wrong species sorry.

 

[aro-discus]

they are doing DNA sexing in asia.

But I'd imaging most real breeders can probably tell just by noticing subtle differences in the fish they are working with. Very hard for the common hobbiest to sex aros unless they pair up on their own, simply because they don't view the same volume of aro's that breeders do.

 

[Koji]

how can you tell if an arowana is a boy or girl?

When you do this you know it's a male.

 

[selki]

I know Jardini are mouthbrooders, dont know about the other Aros. So the male Jardinis should have a bigger jaw.

if jardinis are mouthbrooder so are the other formosus.

 

[ausarow]

the others ( asians) are mouth brooders yep, its just some have said that with jardini its the female that holds the eggs after spawinging them. i did see a vid recently where a female asian got stuck into the eggs on an asian arowana spawn and the male did too but i am not sure if she was just eating them. in nature and behaviour there are sometimes differences to the standard rule.
they have to mind the eggs or they would sit on the bottom and all be preyed upon.
spawners that let eggs into the current have many more eggs then theres others that lay and stay around.