A smoke test is a very good way of finding a leak in a DWV system.
If the smoke come out of a wall or, light fixture you know where to start hacking into a wall or ceiling...
Have lots of people looking for the smoke.
The first places it shows up are where you want to know, not just a room full of smoke...
Before doing a smoke test I would look at all obvious possibilities.
Dry traps, toilets that move, or, rock, inspect pipes in open locations for cracks and holes...
You also can do a pepermint test much cheaper with a few friends to see if it is indeed a DWV system leak.
Get a bottle of peppermint oil from the local druggist, or, health food store... (Note: Large chain phamacies are kinda useless) get a bunch of people inside to sniff and try to locate the first whiffs of the peppermint. Have one person outside mix the peppermint oil into a gallon of hot water and pour it in the roof vent(s) then stay outside so he doesn't carry the smell inside and contaminate the test.
If you get the peppermint smell inside you know you have a DWV system leak and hopefully you can locate it. If you can't you know the visual smoke test will yield results.
If there is no smell of the peppermint I would look for other sources of the smell than the DWV system. A smoke test would probably be a waste of money.