PC133 CAS3 will downclock to PC100 speeds. Usually, it will run at CAS2 when downclocked to 100 MHz. It depends on the DIMM's firmware settings: if those settings allow CAS2 at 100 MHz, your system will use it that way - assuming all other sticks allow CAS2 at 100 MHz.
I don't know of a utility offhand that tells the CAS speed it's actually running at, as opposed to the CAS rating at the maximum speed. It's a good bet that it is running at CAS2 in your machine.