The verify also sees if SQL Server can read the file so if the file hasn't been fully written to disk I don't think you can say it's a valid file that SQL Server can read from. Verify also reads some of the header and the header also has the date/time completed. It seems that writing the file header would need to be completed after the backup has completed if it includes that date/time.
But along the same lines as what RVSC48 said, the only assurance that it's a decent backup that can be used is by restoring it.

Sue