I trust Steve Gibson and I have listened to his podcast for years. Without trust, his tool does no more than what any other internet citizen can do, so even if you don't trust it, it's a trial by gentle fire.

Personally I recommend listening to security now podcast. But then again I've worked in IT for 15 years so it's kind of my thing.

ShieldsUP is fine. Also check out: https://www.routersecurity.org/testrouter.php

You could also just port scan yourself with something like nmap.