Mutley, use whatever method to find the leak, that Building Control did!
It depends how the pipe is .... if there's a vent , where is it? Or is there an air admittance valve, or what?
If you use a proper bung down inthe inspection chamber/manhole, you could put a smoke pellet upstream of it, but smoke would go out of a vent rather than any leak where water seeps out.
If you flush a loo the pipe will fill up and not show much. You'll have 6 or 9 litres above the bung, that's all.
Soil pipe isn't designed to take several metres head of water so I wouldn't really want to fill the whole thing. Frop memory the air pressure test is only a few millibars.
If you try to pressure test it, you'll have to bung the vent. I doubt smoke would get out of a weep even if you did bung the vent.
So - back to whatever the BC man did!