If you pull the hose that goes from the vacuum booster to the intake maifold off the vacuum booster and your idle goes up that is one indication your booster is ok. Another way is to push your brakes down with the car off, then hold the brake pedal down and start the car. The pedal should move when the car starts because vacuum pressure is assisting the brakes.

Usually when master cyl. goes bad it leaks on the booster, sometimes (very rare) into the vacuum booster. Just a small amount of fluid under the mounting point of the master cylinder/vacuum booster is not bad, just keep an eye on it. The master cylinder should be replaced if you brakes are mushy and you have ruled out other brake components or the fluid leak spreads.