with most killis, though i'm not sure about this particular one, the males will be more colorful and have a more distinct pattern, whereas the females will have a duller color and any pattern will be less distinct
Sexing is easy: the male is much more yellow and usually has a more noticeable extension in the middle of his caudal fin. Females are more greyish, and rarely have the red areas males can.
The golden wonder killi is a colour form of aplocheilus lineatus. As it has already been said I think in previous posts sexing is by more colour and longer fins in males.
I prefer the natural fish as opposed to the gold form as i think it looks far nicer.
They are also easy to breed as they are adhesive spawners and will happily breed in a community tank. It is easy to harvest the eggs with a small breeding tank and a breeding mop, java moss or indian fern. I have managed to do it so it must be easy.
I would also suggest a tight fitting lid to save on dissapointment as they are very good jumpers.