David,
Regs are all based on PSSR (pressure system safety regulations) for commercial stuff. Getting hold of a copy of the Model Boiler Test Code will give you a bit of interesting reading and insight. I am pretty sure copies are posted on the net or you can buy a copy from your local model engineering society. I think they cost our club £1 a copy from the federations and we sell them to members at cost.
If the engine is all assembled I would just pump it up to 1.5 X WP, hold it for 5 or 10 mins and look for leaks in the critical places (inside firebox, stays on the outer firebox, smokebox tube plate). It's not unheard of for regulators to leak on a cold hydraulic test so don't necessarily expect to just pump it up to pressure, stop pumping and for the pressure to hold. It is not necessarily a failure if there are leaks and you have to keep pumping to maintain the test pressure – what is important is where the leaks are and the implications of such! You could if you wanted go to 2 X WP but if it's a copper boiler particularly and it's still relatively soft (although if built 20 years ago it's likely to have some age hardening) you can do unintended damage particularly if you don't know for sure from documentation what the WP was actually designed to be! Taking up to a higher pressure does not automatically make it a better, safer test!
If you want to check if it's copper or steel a quick and easy rough test is run over it with a magnet! If the inner firebox and tube plate are non magnetic it's obviously not steel and likely copper (doesn't rule out some grades of stainless though – stainless boilers are not testable or certifiable under the club scheme in the UK!). No real substitute for the mk1 eye ball though and a good careful visual exam.
Good luck and don't blow yourself up!
Paul.