How to test upgrade availability with drush exit codes? I need the test to only do a backup before an upgrade will take place. 
I use version 8.01 and I use the following command: 

drush up -n


This seems to return zero when there are no updates and 75 when there are, but I need to be sure about this. 

Apparently 75 means user abort. Apparently on an up-to-date site, the upgrade process is not interrupted by the "-n" because there is nothing to upgrade and it exits normally.

On an outdated site "-n" will cause the upgrade process to be interrupted and 75 exit code is returned. Please confirm this reasoning is correct, so that I can use it in my script.
The exit codes are defined in includes/drush.inc. Your logic seems sound, but I can't verify that is correct/always will work.