I’d heard about this a while ago from a Land Rover trainer, but had forgotten the details.
The quick version. Set you multi-meter to read DC voltage, put one probe in the brake fluid in the reservoir and the other to a good ground. If it reads more than 0.3V, you need to flush the system with fresh fluid.
The Disco shows 0.18V, so is good for checking again in another 6 months.
The Dakar shows 0.5V, so I’d better get cracking on that one (it’s the easier one to do as well – no fancy ABS system to deal with).