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Well there are a few ways. First, the shipping BIOS version is typically printed on a small label placed either by the S/N on the outside of the box and/or underneath the MOBO close to the CPU backplate.
Otherwise once you boot into the BIOS it's listed there <or> you can use CPU-z and look at the 'Mainboard' tab. CPU-z - http://www.cpuid.com/softwares/cpu-z.html
'Mainboard' tab (note the BIOS section):