I have been looking on ebay to get a feel for the prices if the worst came to the worst.
I can only find these abs fittings for the rear - do the fronts have them?
As Lagdti said the magnetic rings are very reliable and it's a lot more likely that you have a dodgy wheel speed sensor.
I've had no dealings with the front magnetic rings only the rears, the rings on the rears sort of fit over the end of the hub on the disc, they are a tight press fit, they look like they are a dust seal for the bearing but they are magnetic.
You can take a resistance reading of each wheel speed sensor if you have a multi meter, it's probably best to disconnect them like Lagdti described.
When I done resistance readings I done it from the multi plug that connects to the ABS pump, you need the front bumper off to access it.
As far as I can remember I got a resistance reading of around 1200 ohms from each wheel sensor when measured from the multi plug, I think it would be a little less if you measured from where the wheel sensor plugs into the harness.
The connections at the plug for the wheel speed sensors for my car are below, I assume they would be the same for your car.
Left Front - pin 46 & 45
Left Rear - pin 37 & 36
Right Front - pin 33 & 34
Right Rear - pin 42 & 43
I didn't have to spin the wheel to get the resistance reading but if I remember correctly I had to swap the leads around, like they were live and neutral if you see what I mean.
I think you spin the wheel if you are trying to read the output from the wheel sensor, to do this I believe you set the multi meter to the lowest AC setting and as the wheel spins you may get a reading flicking up, I tried this but didn't get a reading on either of the meters I tried.