If you are running version 5.0, then the serial number will show in the "show version" output.
This was a change added in the version 5.0 software.
If you are running version 4.1, then for the IDS-4235, and IDS-4215 you would have to physically look at the stickers on the chassis. Version 4.1 did not have any commands for showing the serial number.
However, for the NM-CIDS you should be able to login to the router (instead of the NM-CIDS) and execute "show inventory" on the router to see the serial number of the module.