Hooking up a pressure gauge

My keg doesn't have a pressure gauge. Should I drill a hole in the top of the keg and fit a brass nipple to the top of the keg, then connect some air hose to a gauge? Or does the air hose that connects to the gauge have to be submersed in the beer itself to get an accurate reading? In other words, do I need to get a reading of the air pressure in the the head space between the top of the beer and the top of the keg or do I need to find out the pressure of the actual liquid inside the keg?

Make one of these: KegPressTester

You can also buy them at northern brewer and other homebrew shops.

Cheers, but these parts will cost more than the keg is worth to import into NZ so I'm thinking about using a 20psi boost gauge and a brass nipple, but either way it's the air pressure and not the liquid pressure I'm trying to check isn't it?

Correct....

Sorry I didn't look at your location and didn't even answer your question.

You should be able to find those parts in a plumbing supply, except for the ball lock connector.

My keg doesn't have a regulator, just a button to release CO2 into the keg. Am I right in saying that if I pressurize the keg to (say) 10 psi just after I have filled it with beer, then the pressure will drop with time as the beer soaks up the CO2, necessitating further pressurization as time goes by?

Yes, you'll need to keep pressure on the beer until it has absorbed all the CO2 it can for the temperature it's at.