The following command worked for Ubuntu and I assume will for Debian, and must be given (as root):
$ sudo dmidecode
or
# dmidecode
For more information see:
How to Obtain the Dell/HP (May be other vendors) service tag in Debian, and don't forget to visit the man page for dmidecode.
If you find you do not have dmidecode installed on your system, update and look in your distribution's repositories.
-- Tom