Hello,
Let me add another, not well-known tool - facter.
Here is an example from my own CentOS 6.5 server.
dmidecode you already said you do not want to use...
# dmidecode -s system-serial-number
4CE25207LN
# lshw |grep -m1 'serial:'
serial: 4CE25207LN
# facter | grep serialnumber
serialnumber => 4CE25207LN
Cheers,