Not easily (without opening the case).
You could always go into the BIOS and have a look, most, when set to auto detect, show the clock speed of the memory. However there may be an issue if the processor you are using is running of 333MHz as you then won't gain anything by running the memory at 400MHz and so it would default to 333. You should be able to set it manually to 400MHz and see if it runs.