Theres a couple different ways. First is the head shape. From when I was starting to learn about bichirs, to now head shapes (other than lower and upper jaws) seemed basically the same to me. However once you see the same fish day after day in your tank you start to noticed subtle differences. I have noticed that once you get to the end of the upper jaw of the polli the head stops abruptly. Where as a retropinnis has a more rounded upper jaw. It's one of those things where it may be hard to see to others, but thats how I've gotten used to noticing a difference between the two. Another way to tell is the dorsal finlets. Palmas Polli have 5-7 and retropinnis have 7-9. You can also look at the eyes. It's not a sure fire 100% way, but from what i've seen most times retropinnis have the speckles around their eyes. If you look at the pic of my retropinnis in my pic and compare it to the eyes of that palmas polli you can noticed that the polli doesn't have those speckles. Once you see the fish enough you'll be able to tell the difference. It just takes time
