ILIAS Developers: Most efficient way to check if user has passed a test

I don't know that there's a better way to do it from the API perspective currently.

I know in ilias 5.3 you have the option to write a SoapAPI Plugin to do exactly what you're asking.
https://docu.ilias.de/goto_docu_wiki_wpage_4573_1357.html

Another option might be to build DB View with all test results and give your external application read access only to the view.