Flippers (pectoral fin)
The Humpbacks scientific name is Megaptera novaeangliae, derived from the Greek mega meaning great and pteron a wing, because of its huge wing-like flippers. These exceptionally long flippers, with knobs along the leading edge, are more like oars, than the paddle-like flippers of the other whales.
The Southern rights have a very large head. It is usually encrusted with large callosities (looks like rocky lumps), on the upper and lower jaws and above the eyes. Sperm whales have a large head too but their body is long and blunt, quite a different shape.
Tail and fin (fluke and dorsal fin)
Position and shape of the fin and tail can help with identification. Southern rights have no (top) fin. Humpbacks have an irregular wavy edge to their tail. And of course Orca males have a very tall distinctive fin.
Blow and blowholes
If you get favourable weather or a close encounter, you can identify much about the whale from its blow. Baleen whales have two holes and two separate blows can sometimes be seen, while toothed whales have one. The actually shape of the blow varies too. Sperm whales have the blowhole on the top left side of the forehead so their blow comes out to that side. And smaller (size and age) whales will have smaller blows.
Colour and markings
Whales are mostly black on the top and white beneath. Many of the big whales seen by whale watchers belong to the baleen group and are distinguishable by the long pleats or folds of skin running from their nose to their bellies, which expand during feeding. The Ocra's have that familiar white patch above the eye and white saddle across their back. Bottlenose dolphins have a blissful grin while the common dolphins have a dark strip from their beck to around their eye.
As you get to know a species of whale you will also start to build up an understanding of it behavioural characteristics. While you may not be sure what species it is, if you notice a different pattern of behaviour, it will give you the clue, to start looking for other identifying characteristics.