Our house is filling up with sewer gas. I suspect two causes:
1. Drain trap in the unused bathroom has dried out and is letting waste line smells in, or
2. Waste water vent that runs through the roof is somehow clogged.

A. I know how to solve (1).
B. What's the best way to check/clear any clogs in waste water vent line?

Anyone know? Thanks for your help.

Jm

First check out what is the real cause.
With 1 that is very easy : just a liter water "down the drain" should stop that problem.

Also check if the main sewerage pipe is not slightly or partly blocked. Use one of the in-line inspection holes.

To check if the vent pipe is blocked : listen at the pipe vent on top, while someone else flushes the toilet.
It must be very easy and clear to hear the toilet flushing.

Success!

;)

The only way I know is from the roof, with a snake... good luck...