Only about 4 years late, but for Googlers, like me...
There is in fact a command to show the memory hardware (even at the time this thread was still active). The command you were looking for was:
Amongst other info from the BIOS, it will show the Physical Memory Array with the maximum memory the system can hold, and then each slot, whether its empty or not.
Other systems evidently also include a command named 'lshw
'. You'll have to use the webpage for lshw, though, because it's not on my systems.