Is this a sure way of determining that the barrel is infected?

First off, one should never burn a sulfur wick in a wet barrel; only do this (which releases the antimicrobial SO2 gas, which we want) when your barrel is well dried-out. This is because the sulfur gas will combine with the water in the wet barrel and form sulfurous acid, which can lead to a bad taste in the wine. That being said, in any container, the only thing I can think of that would extinguish a burning sulfur stick would be the following: lack of oxygen in the atmosphere in which it’s burning, or a wet or defective sulfur stick. If I’m understanding your letter correctly, you’re telling me that when you lowered a burning sulfur stick into the headspace of your barrel, the stick went out. If it was due to a lack of oxygen, that’s actually a good thing and not a sign of an infected barrel. It means you’re doing something right with your barrel-storing method; having an oxygen-free environment will help prevent bacteria and yeast from gaining a foothold in your stored cooperage, hopefully enabling