I would like to test my brake fluid to see whether it needs replacing rather than spend the time and money on it unnecessarily. I've seen inexpensive brake fluid testers on the market which work on the basis of detecting water percentage based on electrical resistance or capacitance. These devices have 2 prongs which you stick into the fluid reservoir and it lights up different LEDs depending on how much water it thinks is in it.
Are these types of testers reliable on to give an accurate reading? I've seen other types of tester but many of them cost more than replacing the fluid, are there other inexpensive but better options?