Let's try this by analogy.

Let's say you have hypothesised the existence of a race of powerful and mysterious beings called "Policemen", with the ability to move quickly through dense traffic. From these characteristics we deduce that, according to the known laws of physics as we understand them, they might use some kind of 'Siren' device to do this. It's a long shot, the scientists say, but the nice thing about sirens is that they are designed to be heard at distance, so it shouldn't be too hard to at least have a listen.

So, in our lab we construct a dish microphone like the FBI use, and we stick it out the window. There's an awful lot of other noises, so the first go isn't very sucessful. Worse, it's frustrating because, for all you know, the sound of a siren may have been right there, but because you've never heard one before, you don't know what it should sound like.

So, you go back to first principles and work out what you do know. And it turns out that a few very clever ideas added to a few more assumptions makes things a lot easier.

The first new assumption is that the siren is going to push all of it's sound into a single note, to make it easier to hear. It may warble, or something, but it's going to spend most of it's time on pure tones, rather than a hissing noise which would instantly get lost in the traffic at the same power. There are lots of good scientific reasons for this, which makes it an 'informed guess'.

So, a new search device is built, stuck out the window, and all kinds of pure tones get picked up by the very sensitive instrument. And they turn out to be the building you're in humming in the breeze, and the whine of the lab fridge, other cars which pass right under the window, Christmas carollers... There's still a lot of non-policeman signals you're having to sort through. It's keeping you from the pub. Something must be done.

Police cars, you realise, should go very fast. Signals transmitted by moving things are subject to the 'Doppler Effect', which if applied to the pure tones we expect from a siren, mean the tones will shift up or down a little in frequency.

Now, this is the clever bit. The microphone has a very narrow focus. Moving things actually pass right across the focus quite quickly. Things which are further away must go faster to pass across in the same time. But the faster they go, the more the doppler effect will mess with the signal in a very recognizeable way. If we combine both, then we can tell if something is very loud, quite far away, and moving very fast. This is exactly what we expect from a Policeman's Siren.

And if you pick up enough sirens, or better, manage to decode the meaning, then that pretty much proves the Policeman theory. And you're bound to get a We're Very Sorry We Laughed At You award, which they give away every year in Geneva.