Easiest way is by looking for patterns on the gill plate. If they're present then it's a jardini if not then it's a leichardti. i think its harder on smaller fish though. think there are other ways like the way the barbel point and the head shape
Australian Arowanas have seven layers of scales on each side compared to five of the Asian Arowanas. The Leichardti has spots on each scale and fins hence the common name, Spotted Arowana.
The Jardini (Pearl Arowana) on the other hand is often passed off as an Asian Arowana since little difference may be seen at their early stages (between 0-8 months) and sold for a lot more than it's worth.
SO be very very careful
Actually there are a few types of Spotted Arowanas. They are originally from Australia (also known as Australian Gold, Aus-Pearl Arowana, Northern or Southern Barrumundi).
My LFS has two leichardti arowanas....they are 300 bucks each. Probably more now.. cause I haven't asked about them since they first came in. They have little pink dots on the scales.. I wish I had the money and the room for one... I think they get bigger than a jardini.