Sure testing a beginner is hard here. But testing someone who learned how to debug means testing their knowledge of the moving parts of the system they are debugging. And this can be explained by the person being tested.

E.g. "I am going to ping an IP on the outside first, to see if ICMP message reaches the outside server. If yes I will check if the DNS server responds, if no we check the physical connection."

That would be an ideal answer, judging less ideal answers fairly is certainly a challenge, but not impossible.