A lot of workshops use a smoke test just as you've described. Another couple of options below:
[quote name='Rosssub said:
To check for boost leaks. Either spray ether (starter fluid) or anything highly flammable around any possible vac/boost leak spots. If any fluid gets sucked in the idle rpm will change straight away.
Or block the intake off at the AFM end:
img]https://s25.postimg.org/or4nozjqn/Fuel_pump_ECU_fix_014.jpg[/img]
Then use an air compressor to pump 10-15 psi into the intake manifold through a vac hose. The system should hold pressure, any air/boost leaks will hiss and be obvious.
If nothing shows up, then as b01z mentioned it could be a dirty solenoid. Most often inside the BBOD, here's a write up on cleaning it out:
http://www.uklegacy.com/forums/index.php/topic/11043-code-66-and-boost-issueshow-to-clean-them-out/
Then also clean out the primary boost solenoid inside the F/L guard, if it's blocked you'll get waste gate spring pressure at 7.5psi: