How to verify an ADB backup file is complete and in a good state? This is not a duplicate of How to verify a backup file from ADB backup. The poster there probably didn't know that an unrooted phone requires -nosystem (ref) and ended with a 0 byte file, where my backup file is almost 8GB. 

Given that adb gives almost zero feedback on it's backup progress and success or failure. How can I be confident that the backup I've just completed is a good one? and that it contains what I think it does? and that it will (probably) work to restore or extract data from?

I have the one android device (Nexus 5), access to Win7 PCs, and ubuntu virtual machines if needed.
If you're on Linux, Adebar contains a tiny little shell script to convert ADB Backups (.ab) to tar files, which you then easily can investigate with any archive manager. Only limitation is: it doesn't work on password protected backup files.

The basic command it runs (everything else mainly is a wrapper for command line options and syntax) is:

dd if=$1 bs=24 skip=1 | openssl zlib -d &gt;${1%%.ab}.tar &amp;&amp; gzip -9 ${1%%.ab}.tar


A command I've found here on SE some time in the past (full disclosure: I'm the author of Adebar) – see here.

As you can see in the quoted command, it requires openssl and zlib to be available.