When whale watching the most obvious way to find a whale is by the huge spout, or blow, as the animal exhales when surfacing. When conditions are calm this blow can be seen for miles as it hangs in a fine mist in the air...a tell-tale sign of a whale below it.
The other giveaway when looking for humpbacks in particular is the fact that these whales love to lift their flukes out of the sea just before diving to help drive them under the water. The flukes often rise high into the air and can be seen for great distances...
And this of course makes for a wonderful photo opportunity as these flukes drive their owner deeper into the water.