Multi card-reader

I have a built-in 4-in-1 card reader on a Dell Dimension system.
Excuse my ignorance, but how do I know which drive letter represents
what type of card?
E.g. I have J, K, L, M as four drive letters.
Without actually having each type of card, how do I know if J represents
Compact Flash/SD/Memory Stick or XD? (Same for each other drive letter:
K, L, M)
Thanks very much.