How do you check ABS Sensors?

[Bob Rossato]

I'm doing some work on a friend's 1989 90.  One of the problems he has is
that when you first start the car the ABS system will activate for the first
couple of times you apply the brakes and then, apparently not happy with
everything, it will shut itself off.  Bad wheel sensor?  How do you check
them to find out which one it is?  Is it just a simple resistance check?

Thanks,
Bob