One of the biggest give aways to a leak in a house is just the sound of the drip, so make sure that all the taps in the houses are off and that there is as much silence as possible, then strand near all high water use areas in your house and try listen for any drip, sometimes climbing into the ceiling can help too.
This would be the first thing you notice if you think you have a water leak problem, your water meter will increase by quite a substantial amount. Keep an eye on this for a few days to make sure it is a leak and not just a spike in water usage. Also try turn all your water off and watch the meter, if the numbers continue going up then there is a leak.
A Geophone is very similar to a stethoscope, it is a listening device that you will place around your house and it will listen for the leakage and ampligy the sound so that you can narrow down where it is. The only floor to this is that if the leak is too big then it will be harder to narrow down.