Run msinfo32 and look for the "BIOS Version/Date" key:
msinfo32
superuser.com/a/1319419
So my BIOS version seems to be the following one:
msi.com/Motherboard/X99A-SLI-PLUS/support#bios
See also: How did I upgrade AMI BIOS from 1.C to 7885v1E?