The anti-lock braking system on a vehicle comes as an additional safety feature available on many modern vehicles. The ABS system is made to help prevent wheels from locking during heavy braking situations, keeping the car from skidding or hydroplaning. The ABS system is summed up of the ABS module, and the ABS sensors at each wheel. The sensors detect wheel speed, and will send a signal to the ABS module to rapidly pump the brakes when it is detected that the vehicle is skidding or has lost traction.
When the ABS system is not working then loss of traction, skidding, and hydroplaning are very more likely to occur under heavy braking situations. The ABS system on most cars is usually designed to give you plenty of warning signs when there is any issue with the system. Being aware of these warning signs, and handling the problem as soon as they come will help ensure that your ABS system and vehicle remains functioning for maximum safety. Below are some of the signs of failing.
1. UNRESPONSIVE BRAKE PEDAL
In most cases, depending on the model of vehicle, when the ABS module fails, the brake pedal may get unresponsive. This is an obvious problem, as an unresponsive brake pedal will not halt a vehicle, or will not be able to do so in an adequately safe pattern. In most cases, this will happen slowly, over time. Usually the brake pedal will become increasingly hard to press until it is no longer responsive.
2. BRAKE PADS REQUIRE MORE EFFORT TO PUSH
When all parts of the braking system are working properly, the pedal should need very little effort. It should be very easy to press down on, and once pressed should have an immediately noticeable effect in slowing the vehicle down. Thus if you start to notice that over time the pedal needs increased effort in order to achieve the same amount of braking force, then that could be a sign of a possible issue with the ABS module.
3. THE ABS LIGHT IS ON
http://www.automobileglobe.com/how-to-know-signs-of-a-bad-or-failing-abs-control-module/

There are more symptoms exhibited when a car has bad ABS module especially on modern cars

Hey Mayor, please bring them up as other folks might be needing it,thanks.

There are also other signs to look out for regarding a bad abs sensor. Here are some other critical symptoms of a bad abs sensor module that I found really interesting and informative.