Just to follow up, out of interest, what do you look for in the samples you take?
The mantra, as in the article that the OP pointed out is
Bright, Clear and Blue (in the case of AVGAS)
It's similar for Jet fuel though of course the dye is straw-coloured.
If you hold a glass sample jar up to eye-level, any solid contaminant will be apparent swirling about, settling to the bottom of the jar. Water shows up in AVGAS as small droplets. Really, it's just the same as the pre-flight samples we all take from the aircraft drains, though the plastic tubes we use aren't as good as a clean glass container.
A dullness to the sample might indicate the presence of microscopic contamination or biological growth (another issue not mentioned in the article) I think this is mostly an issue with Jet fuel, rather than AVGAS and is a consequence of having water contaminant. We don't have a test for bio contamination but keeping the fuel as water-free as possible is a good preventative measure.
Of course, apart from keeping everything clean the best thing to do is to use the fuel and keep replenishing - very difficult at this time of year!
Incidentally, the article mentions the use of a white bucket for checking fuel samples. We use a clean glass jar or a stainless steel bucket with a static line attached to it.
TOO