There are a number of ways to tell the difference between sharks and dolphins. The easiest way is by examining the characteristics of the dorsal fin's shape; shark fins have a straight trailing edge, while dolphins' fins curve sharply backwards at the tip. According to Surf Look, another way to identify sharks is by noticing that their tail fins are vertical, while dolphins' back fins are horizontal.
Sharks' tail fins sometimes break the surface and trail along behind their dorsal fins. Generally, if more than one fin is breaking the surface of the water, the animal is quite likely to be a shark. Movement is another way to tell the difference between sharks and dolphins. Dolphins tend to move in more of an up-and-down manner when they swim. Sharks tend to move in more of a side-to-side motion.
Upon close inspection, it is easy to see that sharks' and dolphins' dorsal fins are attached to their bodies differently. Dolphins' fins emerge smoothly from their backs, while sharks' fins feature a triangular notch on the backside of the base. This feature visually separates this portion of the fin from the surface of the shark's back.
If the fin spotted matches the characteristics of that of a dolphin, there is also a chance that it can belong to a porpoise or whale.