In xterm, enter 'df -h'
You'll see several filesystems, including /media/mmc1/ and /media/mmc2/.
These are your SD cards (if you have two). /media/mmc1 is the external slot, mmc2 the internal one. 'df -h' will show the size of the filesystem, which is how I figured out which was which the first time (one of my cards is much larger than the other)