Here are some options.

https://www.amazon.com/s?k=brake+fluid+test+strips&crid=ESGX75BR7HYA&sprefix=brake+fluid+%2Caps%2C115&ref=nb_sb_ss_pltr-data-refreshed_5_12

Basically they are looking for moisture. Moisture is a concern due to incrasing the possibility of water vapor causing the brakes to not work after an event where the cylinders or calipers get heated up and corrosion inside the cylinders, calipers, and brake lines. The VW is prone to this due to the vented cap. It does help that the reservoir is inside the cabin. It used to be common for mater cylinder reservoirs to have a diaphragm as part ot he cover that would keep air out and move down as the brake fluid level dropped.

Changing brake is really a low cost task and frequently opening the bleeders will help to prevent them from rusting in place. The Vanagon brake system is simple and cheap. Modern cars with ABS and other brake enhancements can have parts failures that quickly go into the thousands. I had the brake fluid changed on my 22 Denali as part of a 50K mile service.

Dennis