Open the cap and look at the inside top of the core assembly. If water is not too cloudy, you should be able to count 2, 3 or 4 rows. If necessary, you may have to drain out an inch or see to see the tops of the rows. Or, you can also stick your finger in there (when cool, duh) and feel number of rows. Count rows front to back or back to front. It would be good for an owner to know what they have.