How can we know the 'human-level error' in actual case?

Hey @for_east,
Welcome to the community. There have been a great many discussions in the Deep Learning Specialization in the past, regarding what to do in the absence of any human-level performance. Here are some posts for your reference:

You will find that these posts share a great deal of knowledge regarding your query, so let me know, if these help.

Cheers,
Elemento