You can use a scanner, or you can just use an ohmmeter. About two feet from each ABS sensor on each wheel, you will find an electrical connection. A good sensor will read about 1200 ohms. A bad sensor can either be shorted(zero ohms) or open (infinity). The sensors are very easy to change, without any tools, and they cost about $90 each.
Oops, sorry! They have gone down in price since I last bought them. They are now about $77.
ABS sensor