Is there a command that shows me if is there in the router an offboard memory?
I tried show diag expecting to see DRAM slot 0 "memory module 128Mb" or something like this. but I dont see, my doubt is should the memory show up there? and it means the memory is onboard or no even an offboard memory will not be displaied.
DRAM is always in the mainboard. They are inserted in the slots at the mainboard.
When you mentioned off-board, you might be referring to router flash or non-volatile memory (NVRAM). Depends on the router model, flash is also inserted in the slot at the mainboard, but the difference of flash from the memory is it is available in PCMCIA where you can plug it to an external PCMCIA slot.
I'm not sure if there is an available command to check the memory (DRAM). Your best bet is the memory combination table of the router model.
For flash, there is a way to check it, see this link http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/cs/csprtf/csprtf2/csmemory.htm