doncan, As you're diagnosing this, remember that the exhaust output from the engine is combined into a common chamber before it's vented out the two exhaust pipes.
A really easy way to see if the smoke is "sourced" before the chamber is to block the smoking tailpipe, and see if the smoke then comes from the other pipe (this redrects all exhaust out of the non smoking pipe. If the source of smoke is in the chamber or either header, then the smoke will statr to blow from the previously not-smoking tailpipe). If your smoke does not change, then the smoke cause is in the muffler assembly itself (condensation, contamination, etc)...
If the smoke is at/before the chamber, the next easiest diagnosis is to pull the right side header (easiest access to remving the chamber and headers anyway...) to see if the smoke is from the right side or possibly the left and/or chamber. If not from the right side, pull the chamber and left header to get to the smoke source.