P. palpebrosus have nice even rows of scales on their backs. The P. trigonatus have very rough uneven rows.

http://crocodilian.com/paleosuchus/paleosuchus.html

This is a very good website to go to. If you go under the describtion link you will see pictures and even more differeces between the two. Hope this helps. If you any other question just ask. I have a P. palpebrosus and don't mind helping if I can.