I wish there was a 100% guaranteed way to determine a failed/bad MOBO. The only 100% is to replace ALL of the components with compatible + known good components. Example: RAM, PSU, GPU is no onboard VGA, and CPU -- and then only in a FULL BREADBOARD which means no headers connected e.g. USB, Audio & Front Panel.
Failure Rates on older systems:
I myself had to grapple with MOBO & GPU failure rates, and up until LGA 1155 would have agreed - 3. GPU & 4. MOBO. Excluding the B2 recall, the MOBO failure rate post LGA 1155 is significantly higher for MOBO than GPU.
However, lately the MOBO failure rate has been leveling-off.
Ask me again in 6 months and the answer might be different.