They are very easy to distinguish. You won't need pictures.
The Asian Red tail cat is blackish grey with white whisker. Generally as they get bigger they turn a weird blue. The red tail also seems to come with age. Its a very slender fish. So it is quite fast.
The south american redtail catfish Is very wide. This is because of its massive mouth that will eat anything and everything over time. Its two toned. Grey on the top with a white belly, and a noticable red tail at any age.
Thats the general descriptions. If you want behavior info let me know.