Having your mechanic check your brakes and brake fluid after every oil change is a wise practice to follow. They'll be able to tell you how well your brakes are performing and whether you need new brake fluid. Most drivers discover that their brake fluid needs to be replaced every four to five years.
Check to see if the fluid in the brake fluid reservoir, which is normally located on top of the master cylinder under the hood of your car, is still fresh to see if it's time for a change. Brake fluid is often light brown in colour, and in some vehicles it is clear (at least when new), but it darkens over time and becomes murky due to water contamination. A better option is to get it moisture tested by an expert and see what they recommend. This service is frequently available at the same location that does a quick oil change.
Brake fluid is just as important for stopping a car as engine oil is for keeping it running, but it doesn't get the respect it deserves.