My understanding is if it has a convex shaped head then Chitala lopis, if it has a convex shaped head without the dorsal fin then Xenomystus nigri, if it has a convex shaped head with a dorsal fin then Notopterus notopterus.
Below are pictures of each for comparison. the order is lopis, nigri and notopterus. Keep in mind though that lopis` jaws continue to grow and extend away from the fishes face.