Since you've already RMA'd the motherboard, this is for future reference only. A checksum error does not automatically mean the BIOS is corrupted. The CMOS battery could have been defective or possibly not fully seated. This would be the first thing I would have checked because if the battery is bad, a BIOS flash will not help.
To answer your earlier question, you would need a DVOM or similar test meter to check the condition of the battery. The CMOS battery is a silvery, flat, disk which you should be able to easily locate on the motherboard. Check your motherboard manual for the proper way to remove it but it's not difficult. In your case since the motherboard was new, you would have to weigh the expense of returning it versus the cost of replacing the CMOS battery yourself. Then again, it might not have been the battery.
That said, one other way a BIOS can be corrupted is if the PC was improperly shut down like if power was disconnected while it was still running.
