To check the ABS brakes you should test brake as hard as possible at low speeds. If the brake pedal vibrates and pulses and make a certain sound the system is working as it should.
ABS operates by automatically reducing the braking force on wheels that are about to lock. When there is no longer any risk of the wheel locking up, the system increases the braking force again. This process is repeated several times per second, which helps you maintain steering control of the car.