This isn’t available out of the box in HAM.
By default, the Asset Audit process resolves the scanned value to the asset record, but it doesn’t store how it was matched, meaning there’s no built-in flag to say it was scanned by serial number versus asset tag.
If you want that flag in your audit report, you’ll need a small customization. Typically that means extending the scan logic so it checks whether the value matched the serial number field, and then setting a custom flag on the audit result record. After that, you can simply report on that field.
The good news is that this has already been documented step by step by the community, so you don’t need to reinvent it. This article walks through the full implementation of enabling serial number scanning and adjusting the logic:
How to perform Asset Audits by Asset Tag and/or Se... - ServiceNow Community
