A backup system ought to be able to handle several scenarios including file deletion, theft, natural disaster and file corruption. [http://en.wikipedia.org/wiki/Data_loss]
In terms of file corruption, the obvious way to check whether a backup is adequate is with a file checksum. If the backup is corrupted, the chance that the backup matches with the original is negligible (unless of course the original is corrupted as well, but let's assume it is not). Is it possible for a checksum comparison to fail to detect file corruption?