How to tell if a fin above the water belongs to a shark or dolphin
A sharks dorsal fin will have a straighter trailing edge while the dolphin and porpoise have a very curved trailing edge dorsal fin.
You can also feel confident in being able to spot a shark fin by knowing the sharks tail fins are vertical and sometimes break the surface ever so slightly behind the dorsal fin. Dolphins, porpoises and whales tail fins are horizontal. Many shark species also have the second small dorsal fin close to the tail that may be seen protruding from the water. See descriptive drawing below.