Detecting a magnet is one thing, but detecting if one is broken is going to require all the sensitivity you can find. Probably a hall-effect sensor that produces a 4-to-20 miliamp output connected to a PLC analog input card. Through many trials, determine if a broken magnet always produces a smaller output than a good magnet. In theory, it should have slightly smaller field. If the magnet is missing, this method should have no problem (analog input should be 4 mA).
I guess it will depend on the orientation of the magnet. Some directions will produce higher fields than others. You may have to turn the plastic molding in a certain direction to make broken-magnet detection possible.
Most likely, good magnets will produce analog signals all over the range, unless your magnets are precisely controlled field-wise.
