[SwedishBus]

The normal method for doing this job is to use a leak-detection machine; basically a smoke-producing rig that pumps smoke from baby-oil (or similar) under very light pressure through a fitted bung. In this particular case, you would fit the bung in the exhaust tailpipe with the engine stopped and the system cold, and give the smoke machine a couple of minutes to fill the exhaust system, then any leak around the manifold area or backwards would be apparent.

Do you have a friendly local garage that has such a machine? They are fairly common (I use a Smoke Pro, but there are several types) and for the sake of a few minutes, it would give you an instant result.

Don't be tempted to try making your own smoke (like the internet says!!!) as you will either kill the catalyst, or kill yourself, in the process of using the wrong sort of smoke.