I would call a plumber. The best way to introduce into the system is through a outside clean out plug. But I have also used the vent stack on the roof. The main thing is to do it from outside the home. That way any smoke you see inside the home is coming from your source of the sewer gas leak.
Last edited by dj1; 12-14-2011 at 12:02 PM.
If the plumber doesn't have the equipment for a smoke test, or has never done one, get a new plumber.
At this point it sounds like you'd be better off getting a pro to do this job. Don't compromise your safety for a 'fistfull of dollars'.
Your roof has a few vents and you need to know which one is what.
But if you have a clean out in the ground, better use that.
That curved pipe on the roof could be to the furnace. The "over your head" statement may be correct. Don't make more problems than you already have. There's no shame in admitting a problem needs a professional to diagnose or fix. It's also good to at least try to DIY.