The dmidecode command line utility dumps a list of SMBIOS specifications to the standard output. In order to get the version number of the currently installed BIOS open a shell and
sudo dmidecode --type 0 | grep Revision
The –type 0 option restricts the output to BIOS specific information and grep fishes for the revision number.
On my X61s Thinkpad the resulting output is
BIOS Revision: 2.19
Firmware Revision: 1.3
Rforge 