Posted this in answer to another member, different problem, but thought it might be helpful to someone else looking particularly for testing brake fluid.

I use a dvom (method), there are several ways to test. The second method in the following link, is an after market type tester (I use dvom, because who doesn't have 1 or 2 around?).
Ramac.com - Auto Parts, Shop Supplies, Shop Tools, AUDI®, BMW®, MINI®, SAAB®, VOLVO®, VOLKSWAGEN® and More!

 

Hhhmmmm. Copy/paste didn't like explanation link.

Here it is =

Ramac.com - Auto Parts, Shop Supplies, Shop Tools, AUDI®, BMW®, MINI®, SAAB®, VOLVO®, VOLKSWAGEN® and More!

 

By the time you take a sample from each caliper, the master and pay for the tester, it's much easier just to change the fluid,

 

I agreed, if you drive and often brake hard, why bother to check? Brake fluid is cheap and easier to change than engine oil, just change it once a year or two year.