DOT 3 and 4 brake fluid is made with alcohol and glycerine as the basic components. It's "hygroscopic", meaning it will absorb moisture from the humidity in the air. I'd recommend flushing the brake system every couple of years with fresh fluid, rather than try to test it. A couple of cans of new brake fluid and a little time spent bleeding the system is cheap life insurance. DOT 5 fluid is silicone-based and doesn't absorb moisture, but that stuff opens up a Pandora's box of problems when it's used in a system not engineered for it!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!