IF you have vCenter and IF you are running the CIMs for your hardware vendor, then you should be able to see the capacity, type and speed of the memory in each slot in the Hardware Status tab. If you don't have vCenter, but you have the CIMs, you might be able to get the info from dmesg. Without the CIMs I don't see that info being available anywhere else.
On server with two Xeon E5620 processors, on Hardware Status tab i see: Memory "Speed: 1333 MHz"
but the specs, this processor only supports 800/1066 MHz memory: