There are two ways of noticing there's a bush fire in the area. One, obviously, is the smell of smoke. The other is that the sky turns a vague orange, which intensifies the closer you are to the fire.
Currently down south (FAR away from here) there's a large bush fire burning, and the smoke has blown up the coast to us. The last few mornings Perth residents have woken up to a haze over the city. Even in the afternoon, when the sky is (as always) that clear intense blue, if you look down near the horizon you will notice the sky looks a slightly ruddy orange colour.
On a more humorous note, the news this morning has a representative from some government health office advising people that breathing problems can be aggrivated by the smoke, so they should keep their puffer with them at all times. This always makes me imagine people carting around puffins under their arm as they go about their daily tasks, but no, puffer is just the Australian word for inhaler.