How can I determine direction of water flow through a pipe?

Ben - It might be a bit more complicated than just tracing the pipe back to the pump itself, but you could borrow a technique that they use in medicine that relies on the Doppler Effect. Now you're probably familiar with the Doppler Effect. It's what makes the sound of an ambulance or a police siren change as it's coming towards you or away from you. When the source of the sound is coming towards you, the soundwaves get squashed together and as a result has a higher frequency, it sounds higher. When it goes away, they're stretched - so lower frequency, lower pitch.

Now, you can also use this method in medicine with ultrasound. In fact, they use the ultrasound Doppler to observe blood flow in a foetus. This is what we could steal to follow the water in your pipe.

So if you put an ultrasound signal in the pipe and then measure the sound, then if the recorded sound is of a higher pitch, then you can infer that it's been compressed by the water, and so that should mean the water is moving towards you, I think. So, you could use it, but I think probably, tracing it through to the pipes is considerably easier.