My AC doesn't work and I'm going to have it leak tested and recharged.

When I've seen a leak test done before they hook up a canister of nitrogen to the fill valve, open the stop and check the pressure/listen for leaks.

Wouldn't think only test the system between that valve and the compressor? Am I right in thinking that unless the compressor is turning nothing will flow through it so this pressure test can't test for leaks after the compressor?

If my thought is right, is there a way to test the rest of the system?

That test checks everything, even if the compressor isn't turning it will still allow the Nitrogen to pass through it. What they should do is pressurise it, shut off the valve and then leave it for a while to see if the pressure drops. Simply listening for a leak will only show up a big leak but not a small one. It's quite possible that you can have a leak where it will work for maybe a couple of months but will be losing the refrigerant very slowly, listening definitely won't find that. Not sure why they use Nitrogen as the first step in the regas process is to vacuum the system to get everything out of it so you could just as easily use air as it's going to be sucked out anyway. Maybe something to do with air having water in it?

I did leave out a few steps to save some typing :P

What I've seen done is:

1) evacuate system
2) pressurise system with nitrogen
3) disconnect nitrogen
4) leave it for a while and check pressure
5) evacuate nitrogen
6) pull a vaccuum on the system
7) leave it for a while and check pressure
8) refill with refrigerant if both tests are passed

I just wasn't sure how air tight the compressor is when it's not running. In my head, if air was getting past the compressor then that might mean the compressor was lacking compression - like when the EAS compressor wears out or when piston rings etc are gone.

The nitrogen will pressurise the whole system - if the compressor was 100% gas tight between its pistons/vanes/whatever type it is, the gas would just go the other way around the system back through the expansion valve in the evaporator.

That's assuming only one side has been connected up to their manifold - if both sides are connected up and opened then you're pressurising high and low sides from the manifold.

I've got a few bottles of nitrogen and all the kit to test and evacuate my own systems, but unfortunately my high pressure nitrogen regulator needs a few bits to make it usable that I haven't got yet.

I think I've mentioned before, but on a P38, the obvious culprit for leaking badly is the top corner of the condenser where the foam blocks retain water and accelerate corrosion. The second place is the suction (big) pipe on the firewall where its pressed against the foam. When its running the suction line gets cold and sweats. Condensation is soaked up by the firewall foam and slowly pin holes the rear side of the pipe. Problem is, even with UV dye in the oil, its not obvious, as said small amount of dye that makes it out also ends up in the foam. Soapy water around that area while pumped up with nitrogen is the best way to check.