Quoted from Nokoro:
I should say that the first sign mine are getting magnetized tends to be irregular returns from the phone scoop. This is followed by the ball sometimes (though rarely) sticking to the trough when it drops in and not rolling down to trigger the opto.
So what's the best way to check if your balls are magnetized? I do have some irregular kicks out of the phone scoop and wonder if one of the balls is magnetized?