the symptoms would be a loss of braking at low rpm or having to press the pedal harder than usual or even loss of power brakes period, or a "service ***" message in the cluster.
the abs engaging at low speed or just before coming to a complete stop would likely indicate one of the sensors is not reading the same as the rest so the brake module thinks a wheel is slipping so it engages the abs pump to attempt to stop the vehicle without skidding (even though you may be moving slowly). the result is pulsing of the brake pedal and loss of brake control for a moment and then a sudden complete stop (or any combination of), you may hear a weird noise associated with it as well which is the abs pump engaging.
as far as checking the abs sensors, if there is no code or abs light on then you need a scanner to see live data when this is happening, the scanner will show all 4 of abs wheel speed readings and one may show 0 and the other 3 4mph or something to that effect. sometimes when this happens it's not a enough of a error to throw a code or light up the abs light but still causes a driveability problem.
if it is a abs sensor it's usually a front one and those are located on the front wheel hubs, behind the brakes.
hence why it is wise to check the hubs because when the hubs wear out it causes the abs sensor to go goofy because the sensor inside the hub is affected by the hub bearing being worn out.
the wheel hub is what has the lug bolts and the bearing that lets the wheels spin, the abs sensor is in a hole in the back of the hub with a single bolt holding it in and a wire that snakes up the upper control arm and attaches to the vehicle harness.
the sensor is magnetic and reads a corresponding magnet inside the hub and when it spins this is how it determines the wheel speed
there really is nothing visual to check, if one was unplugged or something you would get a abs light. some people say they can get rusty and that might be a cause, but if you have rust inside the hub then it's probably time for new ones anyway because bearings and rust do not mix well
