Not sure if this is the right section, moderators, please move if not.
I picked up a tool sales catalogue from Kingchrome that had a new item for me, a brake fluid tester for just under $50.
I checked the item on Ebay & found I could buy, what looked identical to the Kingchrome tester for $8.85 posted to my letterbox with tracking.
It measures the moisture content as a percentage, with indicating lights for 1%,2%,3% & >4%. For the first three indicators, "change of liquid is recommended". When the >4% light appears "Urgent change of liquid is necessary Danger"
Now we all know that the usual recommend period is two years for a brake fluid change, but this is just an average time regardless of humidity levels in your area. Two of my cars showed 1% moisture, the other 2%. I have no idea why they are different as they were all due about now. Naturally the Golf which shows the highest reading will be done first.
Here's a link to the Tester & the seller I purchased from.

Portable DOT3 DOT4 Brake Fluid Tester Moisture Water Test 5 LED Compact Tool GL | eBay